Sistema di Supporto alla Condotta (SSC)

SSC is a low-cost ATC/train protection system which was installed on many railways in Italy from 2005 on. SSC is based on a type of transponder originally developed for road pricing applications. The transponders are usually attached to the signal masts. A microwave signal is radiated from train-borne equipment. When a train passes a transponder, the microwave signal is modulated according to the signal aspect. The modulated signal is interpreted by the train-borne equipment, and a braking curve is computed. If the train is travelling too fast, the brakes are applied automatically.

On the more important mainlines in Italy a continuous system with coded track circuits is used for cab signalling and automatic train protection, and there are also other train protection systems. Nevertheless, SSC has seen widespread application.  

General Electric won a contract with the Stockholm area transport agency to resignal a peripheral light rail line in the southern and western Stockholm suburbs. GE signalling has also been installed on a new light rail line linking Alvik and Solna. Installing SSC is part of the contract. The resignalling work on the existing line was running several years behind schedule. On the new section only basic signalling functions were working, and the SSC installation was not active. It is not clear how much of the trouble was actually caused by the SSC part of the system. The SSC train protection as used in Italy seems to be robust and rugged, but the Stockholm transit agency has demanded additional functionality and radio infill.

Notwithstanding the radio infill there are lots of ugly transponder masts at many locations between the fixed signals on the Stockholm area light rail lines. .

The reason for using such an advanced system on a relatively low-speed tramway is not obvious. The original simple SSC as used in Italy - or some other basic train protection system - should certainly have been able to do the job on the Stockholm light rail system!     

During a half-year service suspension in 2017 SSC was installed (and other signalling modifications undertaken) on all private right-of-way sections of the peripheral tramway. Part of the route was reopened after a closure of only four months. It then soon became evident that the SSC was not working properly. Emergency braking was often suddenly applied by the SSC for no obvious reason. It was suspected that microwave communication between tram and transponder might be influenced by external radiation sources. The radio infill WAN system was also malfunctioning and causing unexpected brake applications. As the brakes of these trams are very powerful, it was found that standing passengers suffered from being thrown to the floor. To try and avoid such accidents, the emergency brake mode of the ATP was switched off since the tram service restarted after resignalling. To compensate for this, maximum speed was lowered to 50 km/h. Automatic application of full service braking on passing a signal at danger was initially retained, but it was found that even these quite heavy retardations threw some passengers to the floor, so the SSC was for a long time used only as a cab signalling system.

The situation in April 2021 is that the automatic stop functions are now in service, but as the onboard component of SSC is still not approved, maximum speed remains at 50 km/h.