Intermittent and continuous automatic train protection

In contrast to road traffic, where the driver of the individual vehicle has the responsibility for guiding his car, rail traffic is externally controlled. This is true for the lateral guidance where the train has to follow the track forced by the rim or the wheels, and the longitudinal movement, where signals impose stopping points on the driver. A signalling system has to be used, as reaction on time is not possible when a driver sees an obstacle. In rail systems, braking distances are too long to rely on driver´s vigilance. All systems described derive their information from the line side signalling system and in almost all systems the commands transmitted to the driver have to be identical to or more restrictive than those given by the line side signals. Especially for high density traffic, the existing block system on railway lines is not always adequate and some form of enhanced, or even moving, block can be introduced to further enhance capacity utilisation. In these cases, ATP/ATC partially or totally replaces line side signals with in-cab signalling and has to have priority over the lateral signalling system. Thus modern ATP/ATC systems can offer operational advantages over conventional signalling systems and at the same time can reduce cost. This paper discusses what now should be called traditional or conventional ATP and ATC systems and ERTMS/ETCS. With the exception of a brief mention of ERTMS/ETCS level 3, it does not discuss transmission based or communications based signalling systems.