CTC-teknik i början av 1930-talet

Under åren 1931 - 1935 pågick inom SJ en utredning om CTC, alltså fjärrstyrning av trafiken, på sträckan Malmö - Trellleborg. I samband med detta inkom till SJ följande beskrivning från Westinghouse. I första hand är det väl CTC-system från Union Switch som beskrivs; Westinghouse samarbetade med US&S och agerade i Sverige via Signalbolaget. Beskrivningen är anpassad till brittiska förhållanden; det talas exempelvis om ground frames (ungefär centrallås för lastplatser mm) och "Tube" or "Underground" railways.

Den CTC-installation vid Stanmore som nämns, blev slopad redan i slutet av 1930-talet, i samband med att banan byggdes om. CTC passade inte in i det nya London Transports standard för signalsäkerhetsanläggningar.  Den konkurrerande tillverkaren GRS ansåg sig ha patent på vissa konstruktioner som ingick i CTC-systemet, och stämde Westinghouse för Stanmore-installationen. Det slutgiltiga domstolsutslaget, som innebar att GRS inte hade något sådant patent, kom inte förrän Stanmore-anläggningen avvecklats.

Dokumentet ingår i ärendet Bbrsi 240/42, som finns i volym Bbrsi EI:13 i riksarkivet, Arninge.



Circuit Code and Time Code Systems

I.          General Survey.
II          The Circuit Code System.
III.         The Time Code System.
IV.        Application of the Cirouit Code System.
V.         Application of the Time Code System.
VI.        The Control Machine and Auxiliary Apparatus
Appendix A.    C.T.C. Definitions.
B         Relay Nomenclature.  Circuit Code System.
C.        Relay Nomenclature, Time Code System.
D.        Code Unit Dimensions


Centralized Traffic Control

Code System


The name Centralized Traffic Control (C.T.C.) is given to a system whereby all points,  signals, switch looks or other functions to be operated at a distant location or series of locations are brought under the control of an Operator at a central point.  The name also implies the use of a special system of control and also of a kind that is distinct from systems usually employed for the control and indication of functions close to the control point.

At the present time two different systems can be employed for the control of functions close to the control point. Firstly there is the type of system using some form of interlocking machine or power frame where mechanical or electrical lever interlocking is provided.  Secondly,  there is the type where direct interlocking between levers is not employed and where all interlocking between functions is carried out by means of  circuits between relays - a Relay Interlocking as distinct from a Lever Interlocking.

C.T.C. .is not another type of relay interlocking, but it is a scheme for the distant control of relay interlocking.

C.T.C. systems can themselves be divided into two types, namely Code systems and Multiwire or Direct Wire Systems.  Each type has its uses and as a general rule the code systems are the most flexible and have the widest field of application.

The first coded installation of C.T.C. was placed in service on the Pere Marquette R.R. in U.S.A. in 1928. Since that date, C.T.C, has continued to grow, and up to july 1933, over 900 miles of road with 800 power points and 2,000 controlled signals were under C.T.C. oontrol in the U.S.A.  Installations have also been brought into service in Canada, England and France, and are pending in other countries,

Originally C.T.C. was intended to supersede the Train Order System then in force on single lines with passing loops, but during the succeeding years the system has been applied to almost every kind of railway layout and every type of traffic.  The installation between Peru & State Line on the Wabash R.R. (U.S.A.) is the longest installation, and here the most distant pair of points is 93 miles from the Control Office. It is also interesting to note that the control machine is placed in the Railway Offioe buildings about 1/2 mile from the railway itself.  The Pennsylvania R.R. have installed C.T.C. between Limedale and Ben Davis, a distance of about 30.3 miles. The traffic between these points is extremely heavy, practically all the passenger trains have fast achedules, and cover the distanoe in one direction in about 30 mins. and in the other in 32 mins. The freight tonnage handled amounts to 86,558 gross ton miles per train hour, and the trains are for the most part fast and run at speeds up to 50 m.p.h.

The recent installation in Franoe at Sartrouville on the Chemins de Fer de l'Etat can be quoted as an example of C.T.C. applied to the control of intense surburban traffic

The introduction of C.T.C. has avoided the construction of quadruple tracks between Houilles and Sartrouville, and only one new track has been added. The third track is signalled for reversable working, and the signals on this track, together with the signals and points in the original two roads, are controlled from an office at the Gare St. Lazare about 14 km. distant.

The Stanmore installation of the London Passenger of C.T.C. to branch lines carrying heavy traffic.  At the present time,  about 96 trains in each direction per day with several additional freight trains are being handled by the installation.

The C.T.C. on the Philadelphia Subway is subjected to exceptionally heavy duty, and it is estimated that ultimately no fewer than 21,000 codes will be sent and received each working day.

These few typical installations will serve to illustrate the diversity of application of C.T.C. Code systems, and it may be noted that where figures are available, savings of between 13 % and 47 % have been realized, and these figures do not include many advantages which cannot always be assessed in cash.

Turning now to the  C.T.C.  equipment,  two separate code systems have been developed, one known as the Circuit Code System and  the other as  the Time Code System. For the purpose of describing the  capacity of the systems the typical layout or "station" given on Fig.l is taken as a basis. Thus  the Circuit Code System is  capable of handling the control and Indication of 81 such stations over a total of 3 line wires. The Time Code System will handle 35 stations over two line wires.       

The control point need not be in any particular position relative to the field locations and may be at either end, in the middle, or at any intermediate point. The position of each field location is fixed by the position of crossings,  junctions,  station platforms,  etc, the most convenient and economical location for the field equipment being found by detail consideration of the scheme. Both the time code system and the circuit code system accomplish the same operating results and  the layout of the control machine is identical for both systems. It should be noted however, that a complete code is transmitted in either direction in 3 1/2 seconds with Time Code apparatus, and in I 1/2 seconds with Circuit Code apparatus.

Before going further it should be stated that C.T.C. does not take the place of the ordinary signal circuits and does not in any way alter the usual safety, protection and reliability of such circuits. Each field location represent a self-contained relay interlocking which may have any or all of the well-known locking features such as approach locking, route locking,  track locking and back locking.

C.T.C. is superimposed upon these circuits to enable the distant Operator to control the movements of trains by signal aspects over the crossings and junctions, but only if such movements are safe. Thus, the Operator may send any code he pleases but the codes will only be effective if the locking on the ground permits.In the case of approach locking, the locking is released by the operation of a release push button in the Office Control machine if circumstances in the field permit, and then only if the time element release relay functions for the full time delay period.

The C.T.C. equipment can be divided into two parts -the oontrol machine and the code units.

CONTROL MACHINE, (Time or Circuit Code).

The control machine comprises a sheet-steel cabinet fitted with a front panel upon which are mounted the levers, push buttons, illuminated track diagram, and the necessary indication lamps.  If an automatic train graph recorder is to ha provided, it la fitted below the levers on the front panel and flush with the wooden shelf desk.  To enable the pen tracks to be more easily followed a miniature track diagram is placed just above the pens.

The office code units are housed inside the control machine cabinet, as illustrated on Fig. 13,  for the circuit code system.

A typical arrangement of the control machine control panel is shown on Figure 14.  An illuminated track diagram is mounted above the levers and shows the relative position of signals, points and track circuits together with their reference numbers. Small white or red lamps indicate whether any track circuit is unoccupied or occupied as on ordinary illuminated diagrams.  The row of miniature  levers immediately below the illuminated diagram are for the control of points and have two operating positions - to the left for points normal, and to the right for points reverse.  White and green lamps plaoed just above each point lever indicate that the points are normal or reverse respectively;  no light on these lamps indicates that the points are not occupying either of their proper positions and that the point detection circuit is broken owing to the point tongues not being fully over or that the point lock is not right home.                
The signal levers are mounted below the point levers and operate in two or three positions.) the normal (signal danger) position in either case being vertical.  As with the point leveras, signal indication lamps are provided above eaoh signal lever; when the red indication lamp is illuminated it la an indication that all signals controlled by the lever are exhibiting the stop aspect.  If either the L or R indication lamp (which may be green or yellow according to the individual circumstances) is illuminated, it is an indication that one of the L or R signals la displaying a proeeed aspect.

The spring return push buttons below the signal levers are usually employed to bring into operation a time element device in the field for the release of points if these become approach locked, sometimes these push buttons are used for other purposes such as the control of the automatic or semi-automatic working of signals or the control of signal lamps or signal fog-repeaters.

The row of spring return push buttons below the release push buttons are termed start buttons and the operation of one of these causes a control code to be transmitted to the field station, and a control code thus started contains only "controls" selected by the levers  and push button located vertically above it.

The cancel button is placed close to the code indication lamps and its function is described in detail later.

Any of the indication lamps, levers and push buttons (with the exception of the start and cancel buttons) described above can be arranged to indicate or control other apparatus in the field for instance, it is not unusual for a lever in the top row to be used as a release lever for a ground frame and its associated lamps to indicate the state of the ground frame levers.


The provision of this instrument in the control machine equipment is optional, but there is no doubt that it forms a very valuable addition to the system.

The Recorder comprises a paper chart which moves along under a series of pens, each of which traces a straight line on the paper so long as the traok circuits are unoccupied. When one of the track circuits becomes de-energised, the corresponding pen will be deflected at the same time as the track lamp on the illuminated diagram is extinguished, and will remain deflected until the track circuit clears again. Thus a permanent record is obtained of all train movements which have taken place in the C.T.C. controlled area and from the Operator's point of view provides a visual record, from which he can refresh his memory, of train movements  actually going on in his territory. A sliding glass panel is fitted above the chart so that notes may be entered on the chart. A typical length of chart is illustrated on Figure 17. and from this it will bo seen that the Operator has drawn diagonal pencil lines connecting the pen deflections so as to check the relative position of the trains.

The chart is moved forward by a simple mechanism controlled from contacts on an electric or other reliable clock and therefore eliminates the elaborate internal clockwork gear whioh would otherwise be necessary. Another view of the Recorder will be found in Fig. 16.

 CODE UNITS (I. Circuit Code).

The code units contain the apparatus whioh generates and decodes the code impulses. Three different units are used in the Circuit Code system for the 81 station equipment, namely, the Line Unit, the Coding Unit and the Storage Unit. The office equipment consists of an Office Line Unit, an Office Coding Unit and an Office Storage Unit all of whioh are usually mounted inside the control machine cabinet as illustrated on Figure 13. The Line Unit will be seen in the top left corner, the Coding Unit to the right at the top, with the Storage Units below; each horizntal row of relays in the storage unit represents the equivalent of a field station controlled from this control machine.The Office Line & Coding Units are fitted with a hinge mechanism so that they may swing outwards to facilitate quick removal and to permit easy maintenance of the lever and push button contacts.  Both these units provided with detachable terminal racks, (such as that for the field storage unit illustrated on Fig. 19) to allow speedy change of unit and to avoid all possibility of wrong connections being made in the wiring. The office storage units are also mounted on hinges but are not generally fitted with detachbalbe terminal racks since maintenance can be carried out on individual relays in this unit without affecting the whole system. All wiring in the control machine is of a permanent nature, carried out in the Factory prior to the machine being despatched. Thus, on reaching the site, the only connections which must be made in order to bring the machine into service are those from the local 24 V battery, the three line wires and the local A.C. supply for the indication lamps - a total of seven connections to control machine terminal board.

The field line and coding Units differ only slightly from the corresponding Office Units and are also fitted with detachable terminal racks, but have shock absorbing arubber feet instead of the hinge arrangement. The field storage unit is illustrated in Fig.19, and it is to this unit that the connections between the C.T.C. equipment and the standard field signals are made. Each unit of like name is of standard design (except the office storage unit), and is interchangeable so that in the event of failure a complete unit may be changed. 

The units are all built on very robust lines, each being constructed of a sheet steel case fitted with a glass front to allow the relays to be kept under observation for maintenance purposes. The relays are all of the same general type, i.e. are all direct current neutral relays, no polar relays being used in the Circuit Code System, (no other apparatus such as preselectors or steppers are employed).

Double contact pieces are provided for each relay contact spring and each contact which may break an inductive load la conneoted to a spark quench circuit to prevent harmful burning of the relay contacts. The spark quench circuits are taken through two indicating fuses so that in the event of a fault occurring in one of the quench units, the lineman may be warned that some of the relay contacts may, in consequence, be without spark protection.

The main function of each unit is given briefly as follows —

1. OFFICE & FIELD LINE UNITS (8 Station Basis).

These units contain the Office & Field series Line epeater relays ORX, ORY, RX & RY together with their associated transmitter relays OTX, 0TY,0TZ, TX, TY & TZ, and therfore handle the actual line impulses.

2. OFFICE &  FIELD CODING UNITS  (81 Station  Basis).

The coding units contain the R & T chain relays which with the line relays either generate or receive the code impulses.  These units also contain the intermediate station determining relays for station selection, and the intermediate control relays which store control or indication impulses until they are passed on to the storage units at the end of a complete code.

Each unit (not generally the Office Storage Units) is provided with the standard type of detachable terminal rack (see Drg. 194 • 25). The general construction of the Time Code Units is similar to those described for the circuit Code Units and here again only D.C. neutral relays are employed in the code equipment.

The main function of each unit is given briefly as  follows  —


These units contain all the relays (including the line repeater relays) which are necessary to generate, receive and decode the code line impulses.


The office storage unit contains the code starting relays for each panel on the control machine and also the lamp indication relays for the point, signal and track indication lamps for each panel.


This unit is provided with a sufficient number of relays to handle the controls and indications for the typical field station or an equivalent amount of apparatus. It therefore contains the final stick relays I3P, IIP and 9P, 12P end 10P, which control the point and signal operating relays, etc;  it also contains the necessary starting and indicating relays required to return indication codes to the office.

OPERATION (Time & Circuit Codes).

Normally apparatus, both at the Office and in the field, is at rest. When the Operator wishes to make some alteration in the field such as to clear a signal or reverse a crossover, he will move the appropriate lever and then press the start button. This last action will initiate a control code which will both select the proper field unit and energize the control relays. If conditions in the field permit, the signal (if he has mowed a signal lever) will clear and in so doing will automatically start the field apparatus to transmit indication code back to the Office to inform the Operator that the signal has responded to his control code.  Similarly other movements of the field apparatus such as the occupation of track circuits, end the position of the points will be indicated continuously on the control machine, and any alteration in their position will always send a code ox codes to the Operator to keep him advised as to the state of the apparatus under his control
steps or impulses,  it is possible  to do three different things, namely, de-energize the X line so giving an "X" character  to the impulse,  or de-energize the Y line to give "Y" impulse, or thirdly to de-energize both the X & Y lines,  so  giving the impulse a "Z" character. code will therefore consist of eight line interruptions each of which may have one of the three different characters X, Y or Z.


Since there is only one line channel to all stations, it is necessary in order to select a given station or a particular panel on the office control machine,  that a code must have some distinguishing feature which will serve to operate the code receiving equipment in such a way that only one field location or one office panel will respond to a particular code. In other words, each code must  have a station or panel selecting feature and this is accomplished in the Circuit Code System, by varying the character of four of the eight impulses. Since each of these four impulses can be given one of the three different characters,  it follows that with this arrangement it is possible to obtain 81 different station selecting combinations, viz. XXXX, XXXY, XXXZ,  XXYZ,  XXYY,  etc. etc. In addition to the station selection steps, it is also necessary to provide a distinction between an outgoing control code and an incoming indication code, and this is accomplished by making the character of the first impulse of every control code Z, and the first Impulse of every indication code either X or Y.

The remaining three code steps in a control code are employed to transmit the controls from the control machine levers to the field station and the corresponding three steps on codes are used to return indications to the office.  In the table given on page 18, a typical code arrangement is  shown, such as would be used for the field station shown in Fig. I. It should be observed that the choice of X or Y for the line check impulse on an indication code enables this step to be used to carry an indication as well as to distinguish the type of code.

It is important that the reader should appreciate the value of this code make-up and have a clear understanding of how its  composition varies with the information it carries, and to assist in making the code make-up clear, a graphic representation of a typical control code is given in Fig. 3. A study of Fig. 4 will also help to show how movements of the control machine levers will vary the impulse characters in a control code, and the operation of signals,  points and track circuits will change the impulse characters on an indication code.

On the right of this diagram a train is represented as being diverted to the siding and the control codes necessary to set the road and  clear the siding signal LB and also to send indications back to the office, are shown graphically in the middle of the diagram, the arrow distinguishing the direction of the codes. The impulses are alloted as on page  18 and it is assumed  that AT and WT track circuits are indicated on the 1st and 5th steps of No.1 F.S.U. and MT and ST on the same steps of No. 2 F.S.U. Thus, when the train occupies the approach track circuit AT an indication code will be transmitted to the office having the 1st step character changed from X to Y to report the de-energization of the AT track relay. It is now supposed that the Operator sends a code to reverse the points and clear LB signal, and he therefore moves both the point lever end signal lever to the right and presses the start button.

The control code which is now sent to the field station contains Y character on the 6th impulse to reverse the point control relay and T character on the 7th impulse to pick up LBHR.  When the points have fully reversed and the KR relay has picked up reverse, an indication code will at once be sent back to the office to inform the Operator. It should be noted that this code contains the "L" Signal Clear character Y on the 7th impulse as well as the Y character for points reversed on the 6th step because the signal does not clear until the LBHR picks up when KR has picked up in the reverse direction.

Thus, it will be noted that in one code both the new position of the points and also the altered signal aspect are repeated and that the condition of AT & WT track circuits are both checked in the same code.

The train now enters WT track circuit and an indication code is sent to the office to indicate the fact.  It will be observed that as the siding signal has now assumed the step aspect this information is also repeated in the indication code (Z on the 7th impulse).

It should now be clear that the composite type of code used in the circuit code system effects very considerable saving in codes and in consequence greatly reduces wear and tear of the apparatus, and the time of line occupation as compared with systems in whioh a separate code must be sent to indioate and control each individual function.

It will be noted, in the codes illustrated, how each code contains the panel or station selection impulses on the 2nd, 3rd, 4th & 8th steps, and how, in the indication code sent back when the train occupies ST track circuit, the panel selection is altered from XZXY to XZXZ when sending from No.2 Field Storage Unit instead of No.l Field Storage Unit.



The code action is generated in the coding unit by the counting chain T & R relays. When once this chain of relays has teen started they will run through a complete cycle of operations provided that their associated checking circuits permit. The checking circuits ensure that, first, the relays associated with each step occupy their proper positions before the impulse is put on the line and secondly that if some fault should occur to prevent the proper code sequence,  then the code action is stopped in so far that as that particular coding unit is concerned.

The chain relay circuit is shown on Fig. 5 and from this diagram it will be observed that each relay when energized causes the relay circuit in the rear to be broken and at the same time to energize the relay in advance, so generating a continuous and progressive sequence provided that the checking circuits (not shown in detail) permit this action.


The relay action for a typical control code is shown graphically on Fig.6. and with the aid of this diagram the relay sequence can readily be followed..

The corresponding relay action in the field unit is given in Fig.7.

Examination of the chart Fig. 7 will show that each time one of the T chain relays picks up one of the transmitter relays is energized and in turn de-energizes one or both of the line relays. Thus, on the 2nd step (see also Fig. 8) when 0T2 is energized  the positive battery will be fed over No.2 code setting jumper to the X bus line to pick up OTX and so de-energizes the X line circuit. Similarly, on the 3rd, 4th & 8th steps the character will be determined by the setting of the jumpers and as shown on the figure the station selection characters will be XZXY. The character of the 5th, 6th & 7th steps is determined by the position of the push button and levers; for instance on the 6th step (when 0T6 is energized) X character will be imposed on the line if the point lever is normal, but Y character will be sent out if the lever is moved to the reverse position. Only one control code can of course be transmitted at a time, so that one set of chain relays and the three transmitter relays serve all panels, each of which is connected in multiple to control the feed from the T chain relays, over the particular starting relay of the panel which is sending, to the XYZ bus lines.

Fig. 8 does not show the R chain relays but it will be clear from the chart in Fig. 6 and the chain circuit in Fig. 5 that after the T chain relay picks up the associated R relay will pick up to step the chain along to the next T chain relay for the next step.

As has already been noted the line relays at each field location are in series in the line, the impulses put on the line by the office transmitter relays will be repeated at each field location, and reference to the chart in Figs. 6 & 7 will show that the field RX, RY line relays and TX, TY & TZ. transmitter relays operate in synchronism with their corresponding office relays. Reference to the simplified circuit diagram on Fig.9 will show that with the master relay M down (for receiving) the transmitter relays TX, TY & TZ will follow the line impulses which are repeated by RX & RY, and so X bus line will be energised for X line impulse, Y bus line Signal. From the chart in Fig. 10 it will be observed that these relays  together with XSD are already energized when T8 picks up. Battery is now fed over T8 front contacts from the Y bus line and over a front contact XSD to the XY field storage unit where the delivery relay D is picked up. The final stick relays in the storage unit, 5 YS. 6XS. 6TS. 7XS & 7YS. (see Fig.ll) are now connected to the front contacts of the intermediate stick relays and since 6X & 7Y are energized,  6XS which is already held by its stick circuit, will be retained energized, the points remaining in the normal position, while 7YS will be picked up to clear the Right Signal. The last chain relay R8 now picks up and clears the line, and when both RX & RY have picked up the slow release relay LO is de-energized. Finally R8 drops away and  so de-energizes the intermediate stick relays in preparation for the next code.

Referring back to the 4th & 8th steps, it will be noted that these are two which are used to determine which of the storage units attached to the coding unit will be selected, & since X, Y or Z can be received on each of these steps it follows that a maximum of nine field storage units can be operated from one field coding unit.

So far only a control code has been considered, but each coding unit and line unit is capable of operating either as a receiver or as a transmitter. Thus, when the field unitse transmit en indication code to the office the cycle of operations in the field is similar to that described for transmission at the office end. The master relay M is energized and completes the necessary connections to enable the field coding unit to transmit and as the chain relays progress the character of the indication code impulses is determined by code setting jumpers and by the position of the signal and point indication relays (instead of levers and push buttons) over a similar set of  chain relays as shown for the office transmission on Fig.8. In the same way reception at the office is carried out on exactly the same principles as has been described for reception at a field location.

Synchronized  action between a  transmitting unit and a receiving unit is obtained by the presence of a check circuit by which the  line at  the receiving station is held open by the transmitter relay last energized until the R chain relay  picks up. Thus while the transmitter relay at the receiving station is energized the R line relays in that line cannot pick up again so that an impulse must register at the receiving station and then pick up the R chain relay, de-energize the transmitter relay and so finally allow the line to close.

This check or line lock action is illustrated on the charts in Figs. 6 & 7, on the 4th step. Here it will be noted that the line is held open by TX in the field while XSD & R4 pick up.  Relay OTX at the office has dropped out but the office chain does not progress until TX releases upon the pick up of R4, and permits GRX & RX to pick up together.

The diagrams  have been simplified as much as possible to try to give the reader a clear understanding of the basic principles upon which the Circuit Code system operates. Additional circuits are provided,  however, which ensure that faulty operation of a unit will not result in the standard outside relays of the relay interlocking circuits receiving controls not intended by the Operator & also so that a faulty unit will not interfere with the correct operation of units at other locations.

The principal features of the System have now been dealt with but there remain several important auxiliary features, some of which assist in the speed and convenience of operating the system and others which must be understood in order to be in a position to deal with a possible failure on some part of the signalling equipment.

Code Lamps.

Two yellow lamps are provided on every control machine one of which is illuminated during the transmission of a control code and the other during the reception of an indication code. The indication code lamp is also connected in such a way that it will become illuminated should an open circuit occur in the line circuit.


If for any reason it is desirable to cancel control codes which have been stored at the Office, operation of the cancel button will destroy all storages which have not already been transmitted.

A fault on the system may become apparent by reason of continued code action, and the code indication lamps will indicate in these circumstances whether the trouble originates on an indication code or on a  control code. If this repeat code action occurs on an indication code, a time delay relay in the field will stop the code after about 25 seconds.

If the control code is at fault, code action can be stopped by pressing the cancel button, and the individual panels can then be tried one at q time in order to locate the difficulty.  If the trouble is local to the particular panel, then other panels can be used is needed, independently of the defective panel.

Control codes can be stopped instantly at any time before completion by operating and holding in the start button. The operation of depressing the start button automatically creates a new storage. Thus in case a code for a given set up in the field is initiated and a change of plans is desired, the code action can be stopped at once, the panel set up changed, and the code immediately sent out upon the release of the start button. In case it is desired to cancel the code entirely the start button should he held in until the cancel button is operated and both buttons then released in the same sequence. This operation will also cancel all other storages.


All indications received at the Office are entirely independent of the control apparatus and are responsive only to changes in the field indication circuits. All indications are continuos and any change of conditions in the field is immediately reported to the Office.

The Operator can, however, check any or all of the indications showing on the control panel, by utilizing the recall feature, which consists of simply pressing the start button of any selected panel.  Since no change of lever position has been made the panel will remain dark, a second operation of the same start button will recall (and re-check) all indications relating to that start button. In the same way if a control code is transmitted and no change in the field apparatus results due either to field locking as already described under the heading of "locking" or to faulty operation of some portion of the field apparatus a dark panel will be obtained. In this case also a second operation of the start button will recall all indications which when obtained will report the position of the signal, points, etc.


Should the start button be operated  to  transmit a control code to the field while an indication code is being sent in,  the control code is  stored and will he sent out immediately the line clears, without another operation of the start button.   Additional storages may be made,  up to the number of start buttons provided on the control machine.  Furthermore,  the start buttons may be operated in quick succession at any time and control codes will follow each other until all control storages have been run through.

In the same way storages are made in the field should the line be busy, and stored indications will be sent in directly the line clears.

Code  Repeat.

The coding circuits are arranged so that should a coding unit be unable to complete the whole code action the repeat action takes place and thus gives the unit in trouble a chance to clear  a difficulty which may be only temporary. The code repeat feature is also of use since it gives the equipment every opportunity of delivering a code which might otherwise be lost. If the fault remains, then the repeat can be stopped either by using the cancel button, or by the Time Delay Relay in the field.
code and re-set the equipment for  the next code.

It is important to note that a long impulse is always used to register information, and a short impulse to stop the counting chain and convey the absence of information.

The seven station or panel selection steps must always contain three long impulses and four short impulses, and by arranging to vary the position of the long impulses in this group, selection can be obtained for 35 stations and panels.

Thus as in the Circuit Code system, the codes are of a composite nature and each code contains the selection impulses as well as impulses for the control or indication of the field functions. The actual formation of the complete code is controlled by the position of the office code setting jumpers and the levers in the case of a control code and by the code setting jumpers and the positions of the local field relays for an indication code.

Chain Circuit.

The line impulses are generated and received by the counting chain relays 1 to 8, the principle of the counting chain may be seen by referring to Fig. 22.  The fourth impulse is shown and relay 4 is shown stuck up over the front contact of the line repeater relay R. The next operation is the release of R which breaks the stick circuit for relay 4 and also applies battery to relay 5 over back contact of relay 3 and front oontact of the relay 4. Relay 4 will remain up for a short time due to the delay action of rectifier 3 and for sufficient time to allow relay 5 to pick up. As soon as relay 5 picks up, the rectifier circuit to relay 4 is broken and this relay then drops away. It will thus be seen that relay 4 is rendered slow release only until it has served its purpose after which it is immediately released.

The next operation is the picking up of relay R. This breaks the feed for relay 5 and energizes relay 6 over a back contact of relay 4 and front contact of relay 5. Relay 5 is rendered slow release by rectifier 2 until such time as relay 6 picks up when relay 5 will release. It will thus be clear that the counting chain action is a continuous sequence.

From Fig. 20 it will be observed that all R relays are normally energized but will be de-energized when one of the T transmitter relays pick up.

The line impulses for any particular station are generated by a back contact of the T relay.  The character of each impulse is determined by the interval of time which this contact holds the line open or closed.  The operation of the T relay is outlined in Fig.21. Normally the T relay is alternately picked  up and released by the odd numbered  chain relays 1-3-5-7 to generate the short impulses.  When selection requires that  the line be held open for a long impulse, positive battery is applied to the stick circuit for T and it remains up until relays 1L & LP release.  In the same way when a long closed impulse is required, relay T must be held down for a "long"  period.  Relay T is therefore held down by means of a circuit which applies +ve battery to both terminals of the relay. The stick down circuit is maintained until relays 2L &. LP release, thus timing the long closed impulse as for a long open impulse.

In this way the series of long and short impulses which forms the complete code is generated by the counting relays and the character of each impulse so generated is determined by the circuits connected to relay T,  which circuits are in turn selected by the code setting panel or station jumpers and by the position of the levers or field local relays.

Before going further the action of the slow release relays 1L, 2L, LB and LBP must be considered and the circuit for  these relays  is shown on Fig. 23.  During code action all relays are energized,  but the relays are timed so that 1L and 2L bridge the time of a short impulse but will not bridge a long impulse.  It can thus be seen that  if the relay R remains  energized for a long impulse relay 2L will release while if R stays down for long impulse relay 1L will release and in this way these relays  when receiving select between short and long impulses.  Either 1L or 2L releasing  will de-energize LP, the function of LP being to add a short additional delay to the release time of 1L. or 2L when one of  these relays is holding the transmitter relay T up or down for a long impulse. Thus the unit which is transmitting provides a small additional delay to a long impulse to ensure that all stations whioh are receiving the code have ample time to release their 1L or 2L relays as  the case may be. The LB and LBP relays serve to protect the unit from line interference and to reset the unit in the event of such trouble.

The code action thus far described may be seen in Charts Fig. 24 & 25.

When no transmission is taking place each coding unit is set to receive a code and before it can transmit, the master relay M must  be energized and Chart Fig.24 shows that this relay picks up immediately the starting relay ST becomes energised. The same principles apply to  the field equipment and the M relay will pick up directly the starting relay is energised by a change in the position of the local indicating relays.

With the master relay up the code will progress to the eighth step - that is, the counting chain will complete one cycle of operations. At the end of this cycle, if on a control code, one of the field stations will have been selected ready for the reception of the information (controls) contained in the next part of the code. If it is an indication code the office apparatus will have followed the transmitting field station and the storage bank; corresponding to the field station will be selected in readiness to receive the information (indications)  in the remaining code steps. Immediately the 8th counting relay is reached a new cycle begins at No.l. counting relay, to transmit the controls or indications to the receiving station. The complete code action is entirely automatic and reference to Charts Fig. 24 - 26 will help to sake the transmitting and receiving notion clear.

It should be noted however, that as there are only 14 impulses in a control code the second cycle of counting relay operations progresses only as far as No. 5 relay when all the controls will have been transmitted and the equipment will set for any further code action.

As the 2nd cycle proceeds at the receiving station the information registered on the long impulses is stored on the intermediate stick relays 9, 10, 11, 12 and 13-15.  This action can be followed from Fig.27; as already stated when a station is receiving the slow release relays 1L and 2L select the long closed and open impulses respectively.  Thus on the 10th step  (which will be a "closed" impulse) if 2L drops away due to the impulse being long relay l0 will be picked up over a front contact of No2. counting relay and will remain up through a common stick circuit over a front contact of SP. Similarly if the 11th step (which will be an "open" impulse)  is long,  relay 1L will be de-energised end in so doing will energise relay 11, to store the Information on this relay. If the 10th step had been a closed short impulse conveying the absence of Information,  then the delay action of relay 2L would have bridged the short impulse,  in so doing would not cause relay 10 to pick up on this step.


The foregoing sections deal with the  principles of the systems and with the Code Equipment itself, and it is intended to  conclude with one or two brief notes which will, it is hoped, assist in the consideration of the application of C.T.C. to any particular stretch of road.

Considering first  the Circuit Code System, it was mentioned earlier in the Description that the allocation of codes given on page 18. was typical,  and that it could be varied at will;  thus, in planning the number of field storage units which will be required to handle the controls and indications at any particular field location, it may be found that there are more signals to be controlled than there are points, in which case one of the units may be connected so as to control & indicate signals on both the 6th & 7th steps

It may also happen that the number of track circuits  to be indicated may total more than can be handled on the 1st & 5th steps of the available code, and the additional indications may then be set on the 6th or 7th code step of any unit which has one of these steps spare.  If no spare steps are available, an additional unit will have to be allowed, and it is  then capable of sending one, two, three or four indications as required.  If it should happen  that after accounting for all apparatus to be controlled and indicated a unit has still an unused code step,  this is set to X or Y by connections on the unit rack and may be regarded as a true spare, since it can be very easily utilized at any future date after the equipment is in service by simple alterations to the external wiring and  the addition of the necessary levers A/or indication lamps on the control machine front panel.

As has been previously explained  in Part II one Field Coding Unit will control from 1 to 9 Field Storage Units.

The extreme cases of a full 81 station system would be 81 locations with 1 line Unit, 1 Coding Unit and 1 Storage Unit at each, or 9 locations with 1 Line Unit, 1 Coding Unit, and 9 storage Units at each.

The overall sizes of the units are shown on Drgs. W.194, 4-6, and they may conveniently be grouped on a suitable rack in the relay hut.

The circuit diagram (Fig. 11) shows the arrangement of connections to the field storage unit,  the diagram on the left showing suitable circuits when using the typical code allocation on page 18. while typical alternative connections are shown for other node allocations such as control of points on the 7th step,  track indications on the 6th & 7th steps,  etc. The list below gives a few of the more usual functions which may be controlled or indicated on the various steps —

1st Step.

Indicate —  Track Circuit (clear, occupied) - Air pressure or voltage (high, low) - Lights (in, out) - Signals (on, off) - Fog, or No Fog - Leakage Indication (clear or fault) - Ground Frame (locked, unlocked) - Traffic Direction (Eastbound or Westbound).

5th Step.
Indicate -- As for 1st Step,
Control —Approach Lock Release - Call-on Signal - Auto or Non-Auto operation of Signals - Fog Repeaters (on, off) - signal Lights  (in- out)-to Cut-in Indications not normally required - Ground Frame (Release,  Lock) .

6th Step
Indicate --  Points  (normal, Open & Reverse) - Signals  (Left or Right Off or All on) or 3-position semaphore 0°, 45°, 90° - (or 2-position semaphore  0°,   90°,  or wrong).  Also as for 1st step.

Control —  Points (N, R) - Signals (L or R Off or All on) and also as for 5th step.

7th step.
Indicate —  As for 1st & 6th steps.
Control — As for 5th & 6th steps.

A knowledge of relay interlocking circuits and consideration of Fig.11 will then enable the number of relays required at each location to be estimated.

All relays and circuits controlled by the final stick relays (5YS, 6 XS,  6YS. 7XS. 7YS) in the field storage unit should be operated by direct current at 24-v. and similarly the indication circuits from the track relays signal and point indication relays,  etc. should be 24-v. D.C. although the relays themselves may be of either a D.C. or A.C. type.


Reference  to page 28 will show a typical allocation of code steps for a station controlled by the Time Code System, but in planning a layout it may be found desirable to use certain steps for purposes other than those stated.

For instance,  it may be found that the number of signals to be controlled exceeds the number of points, or that there are more track circuits than can be accommodated on the 9th & 11th steps of the available  codes and in these cases code positions other than the typical ones given on page 28 can be used for these functions.

For example, steps 9 -11 can be used for signal control and Steps 12 & 14 can be used for track circuit indication. Many of these variations are shown on Fig. 28,  but this by no means exhausts the possible use of the various steps, and some further suggestions are given in the table below.

Indication Code Step 9 or 11.

Track Circuit (clear, occupied) - Air Pressure -Voltage  (high,  low) - Lights  (in-out) - Signal (on-off) -Fog or No fog - Leakage indication (clear or fault) -Ground Frame (locked-unlocked)  - Traffic Direction Eastbound or Westbound).

Control Code step 13.

Approach Lock Release - Call-on Signal - Auto or Non-Auto operation of Signals - Fog Repeaters  (on-off) -Signal Lights  (in-out) - to cut in indications not normally required - Ground Frame (Lock-Release).

Control code steps 9 & 11.

Points (N, R) - Signals (L or R off or all on) - also as for step 13.

Control code steps 10 & 12.

As for steps 9 & 11, and step 13.

Indication Code steps 13 & 15

Points  (Normal, Open &. Reverse)  - Signals  (L or R off or all on) - 3-pos. semaphore 0°,  45°, 90°.  - 2-pos. semaphore 0°, 90° or wrong - also as for Steps 9 or 11

Indication Code Steps 10.  12 & 14.

Signals (L or R off, all on, or wrong). Also as for steps 13 & 15 and steps 9 or 11.

Thus the controls & Indications may be distributed to give the minimum amount of code apparatus at each location, and  if it is then found that  there is still a code step unused,  this can be regarded as a spare,  and it can  be very easily utilized at any further date after the equipment is in service by simple alterations to the external wiring and the addition of the necessary lowers &/or indication lamps on the control machine front panel.

The available station call signs are divisible into the following groups ——
234    245    345    256    356    456    267 
235    246    546    257    357    457    268    
236    247    347    258    358    458        
237    248    348                    

367    467    567    278    378    478    578    678
368    468    568                    

and any number of field storage units from one up to  the maximum number  in the group can be controlled from one field line-coding unit having the code of the first two figures of
the group.

E.g.    F.L.C.U. No.23 can control 5 field storage units
            «       45    "        3    "
            "       56    "        2    "
            "       27    "        1    "

 It should also be noted that one combination, say 234, may be used at one location and the remaining combinations of the group at other locations with separate line-coding units.

The overall sizes of the units are shown on Drgs..W.194, 21 to 24 and they may conveniently be grouped in a suitable rack in the relay hut.

The circuit diagram (Fig.28) shows the arrangement of connections to the field storage unit, the diagram on the left showing suitable circuits when using the typical code allocation of page 28 while alternative connections are shown for other code allocations, such as control of points on the 10th - 12th steps, track indications on steps 10, 12, 13, etc.

A knowledge of relay interlocking circuits,  and consideration of Fig. 28  will enable the number of relays required at each location to be estimated.  All relays and circuits controlled by the final stick relays 13P, 9P, 11P. 10P. 12P. should be operated by direct current at l6-V and similarly the indication circuits from the track relays,  signal & point indication relays etc. should be 16-V. D.C, although the relays themselves may be of either a D.C. or an A.C. type.


When the number of field locations, and the number of units at each location has been determined in accordance with the principles outlined in Part IV. or V., the layout of the control machine front panel can bo considered.  It is obviously not possible to give an exact size for the control machine as  it may vary on account of many different factors. As a guide, however, it can be taken that the minimum dimensions are

Length    3'-0".    Height 5'-4".      Depth 1*-7".

The usual horizontal spacing of levers (or panels) is 2 1/2" or 3", but this is sometimes increased so that levers may for ease of manipulation be placed as nearly as possible below their relative positions on the track diagram above  them. Then again, if for  instance two pairs of points are controlled from one panel (i.e. one start button)  both point levers could  be placed in the top row. Space also is often allowed for further extensions of the system,  in which case the front panel is drilled and provided with suitable plugs which can be removed when the levers are fitted.

Thus it will be seen that although the height and depth of the machine will not  in the ordinary way vary from those given above,  the length will depend upon the actual scheme, 5 feet probably being about the average.

There is another factor which may alter  the overall dimensions,  the provision of an Automatic Graph Recorder.  When this instrument is fitted, the desk shelf is made several inches wider to acommodate the sliding glass plate over the chart.  The weight of a control machine complete with Office Line, Coding and storage Units  la not great and any sound, well-built floor should carry a C.T.C Control machine of practically any size without strengthening or other special provision.

Referring now to power supply,  the office and each field location will require a direct current supply, preferably obtained from storage batteries trickle charged from a metal rectifier set. The voltages required are 24 for the Circuit code system, and 16 for the Time Code System.  An additional battery and trickle charger is required at the office to supply power for the line circuit.  he capacity of the local batteries will of course depend upon the number of unite at the location and also upon the number of other relays to be supplied from the battery. As a guide, however,  it has been found that batteries having a capacity of 75 to 100 ampere-hours are sufficient.  For trickle charging these a combined trans former-rectifier set is recommended such as  the B.P. type.

Constant current is maintained in the line circuit and the voltage is increased according to the number of line relays required, and to the resistance of the line wires. The drain on the line batteries is small,  so that a line battery of quite low capacity, say 20-25 A.H. is sufficient.

Storage batteries are recommended in order to maintain a constant D.C. potential, as also to carry over during a failure of the main power supply.  It often happens, however, that a power supply failure would also cause a shut down on A.C. relays and other apparatus, and in this case the only function of the battery is to prevent wide fluctuations of D.C voltage. If it is found that the supply voltage can be regularly maintained within narrow limits it would be possible to dispense with batteries and supply the D.C. for line and local purposes direct from rectifier sets.

The control machine indication lamps are fed direct from an indication transformer which may form part of the control machine equipment or may be placed in some other convenient position.  If the C.T.C. installation is operated on direct current, and the field apparatus (signal relays & track circuits, etc.)  are not de-energized in the event of a power supply failure, then a cut-over relay is provided at the office to transfer the indication lamp supply from the transformer to the  local battery.

It la usual to carry the line wires on poles rather than in cable,  partly since a pole  line is generally less costly, at any rate for installations over 5 or 6 miles,  and partly as the possibility of trouble due to inter-line capacity  is reduced to a minimum. The  line  construction should be such as to reduce possible leakage losses to a minimum,  especially on long circuits,  adequate lightning protection must  be provided,  and sectionalizing and station bye-pass switches and resistors are desirable to enable a field  location to  be switcher out of circuit in the event of failure,  or for maintenance purposes.  The wires can be carried on the same poles as other signalling circuits and  it has been found that C.T.C. lines give practically no interference on any adjacent telephone circuits.

In some  installations however,  notably those on  'Tube' or 'Underground' railways, cable must be employed for the line circuit and special precautions are then taken to balance out the effects of capacity between lines, either by the use of special code units, or by other means.



Office.  The building or room which contains the control machine in a C.T.C.  installation.  The machine may be placed in a signal box, Train Controller's or Dispatcher's Office,  or other convenient place,  not of necessity situated close to  the Railway.

Field Station.   A concentration of signals and points to be controlled by the Operator at the office and located at some distance therefrom.

Control Machine.  The Operator's Office equipment comprising the necessary levers,  indication lamps and relay units to enable him to maintain control of the signals and points at each of the remote field stations under his supervision.

Code Units. The units which contain the code relays that generate and de-code the line impulses.

Control Code.  An outgoing code from the Office Control Machine containing "orders" or "controls"  to effect some alteration in the position of the points ex signals at one of the field stations.                                                                                   

Controls.  Those impulses in the complete code which are used for the control of point end signal relays in the field.

Indication Code.  An incoming code from one of the field stations containing "indications" of the condition and positions of the field signalling apparatus.

Indications.    Those impulses in the complete indication code which repeat and indicate to the Operator the positions of the field apparatus.

Impulse or Step.   An interruption of one or both of the normally closed (or energized) line circuits.

Impulse Character    The character of an impulse or step refers to the line or lines opened on any step, i.e., X line opened, or Y line opened, or Z both lines opened.


Circuit Code Relay Nomenclature.

ORX.  ORY.   RX &  HY.   The Line Relays ORX. ORY,  RX & RY are known as repeating
relays. That is, if a station is receiving, its line relays repeat the impulses which are on the line and if the station is transmitting its line relays repeat its transmitter relays

OTX.  OTY.  OTZ. TX. TY & TZ.    The transmitter relays OTX. OTY. OTZ. TX. TY & TZ indicate their function.  If a station is sending,  they generate the impulses  and  transmit  them to  the line and if a  station is receiving,  they transmit the impulses from the line to the coding unit.

OM. M & M1   The relays OM. M & M1 are known as master relays.  They energized if the station in which they are located is sending, and de-energized if the station is receiving.  Their purpose is,  therefore, to determine if the station will send or receive

0LC &  LC.   The 0lc & LC relays are slow release relays and are known as line closed relays since they receive energy when the line is closed. They serve the purpose of bridging the open periods of the line. They have an additional function of cutting a station out if, for any reason, it cannot receive an impulse.

OLO  &  LO.    The OLO & L0 relays axe slow-acting relays and are called  line open relays since they receive energy when the line is open.  Their purpose is to bridge the closed periods of the line during a  code,  and to serve as a lock so  that a station cannot cut in on the line when it is busy.

OLOS & LOS     The OLOS & LOS are line open stick relays. Their purpose is  to ensure that other stations will be ready to receive before a station starts transmitting.  That is, a station may receive with its LOS up but it may not transmit.  These relays are also slow-acting.

0T1.  Tl.  0T2.  0T3.  0T4.  T4,   etc.    0T1 & Tl are the chain transmitter relays.  Their purpose is to prepare a station, on successive closed periods of the line,  for the following impulse regardless of whether the station is sending or receiving.

0R2.  R2.  03R.  R3.  etc.   These relays are chain release and progression relays. They receive energy on successive open periods of the line and serve to release the T relays of the chain as well as the transmitter relays of the line unit.

XSD. YSD. & ZSD.    These relays are station determining relays and pick up
on the fourth step of the code and on the impulse indicated by the prefix letter. Their name indicates their function.

D   The D relay is known as the delivery relay.  It picks up at the end of each control code and  delivers the functions stored in the intermediate stick relay to the final stick in the field storage unit.  It is dropped when the LO drops.

OD  &  0D1.    These are office delivery relays and operate on the last step of an indication code to deliver information to the indicating relays.

SA  &  OSA.   The SA & OSA relays are known as starting or storage relays.  If the line is not busy at the time they are energized in order to initiate a code,  they serve as starting relays and if the line is busy when they are energized,  they serve as storage relays. That is, they store their starting circuit until a time when the line can receive the code which they initiate.

SB & OSB.    The SB & OSB relays are also starting relays. They pick up  after the SA or OSA and connect the code,  which is set up to the line and coding units so that it will be


Relay Nomenclature. Time code system.

R     The R relay is known as the line repeater relay. This relay repeats the line impulses at each station for the purpose of operating the station coding equipment.

T     The T relay is the line transmitter relay and transmits the code to  the  line.

1L     1L is the Line open alow release relay. At the transmitting station this relay produces the long "open" (odd numbered)  impulses  and at  the receiving station it causes these impulses  to be registered.

2L    2L is  the Line closed  slow release relay. At the transmitting station this relay produces the long "closed"  (even numbered)  impulses and at the receiving station it causes these impulses to be registered.

LP    This relay is the slow release repeater of 1L and 2L relays.  On transmission this relay produces the time element margins necessary to ensure registry of the long impulses at  the receiving station.

LB  &  LBP.     LB & LBP are the slow release impulse bridging relays.  These relays in combination bridge the normal open and closed periods of the line to permit normal code action.

F    F is the first group selecting relay, which operates on the impulse number  by which it is prefixed,    2F,  3F, 4F

G    G is the second group selecting relay, which operates on the impulse number by which it is prefixed, 24G,  25G, 26G  etc.

S    Relay S is the Station selecting relay,  which operates on the impulse number by which it is prefixed, 234S, 235S, 236S,  etc.

FP  This relay repeats any one of the five F relays in the office storage group.

GP   This relay repeats any one of the fifteen G relays in the office storage group.

SP   This relay repeats any one of the thirty-five s relays in the office storage group.

MEP   MEP is a repeater of M and B which connects the office code selecting relays ST, G and F to the coding unit for transmission of the F selecting impulse.

AK,   TK,  NWK,   RWK,   LHGK,  RHGK,  RGK   These relays are known as the indication stick relays.  They directly control the indication lamps and train graph pen magnets.

MSP.   This relay is a repeater of M and S which connects one group of wayside indication circuits to the coding unit for the transmission of indications from the field to  the office.

9P,  10P.  11P,   12P,  13P.    These are the final stick relays which are picked up at the end of the control code if the step of corresponding number is long.  They are energized over the D relay and the intermediate stick relays 9, 10, 11, 12, 13 - 15,  and control the relays in the external circuits for point and signal  control etc.




Sidan uppdaterad den 17 december 2005