Train protection system

A train protection system is a railway technical installation to ensure safe operation in the event of human failure.


Berlin S-Bahn train stop in its engaged (left) and disengaged (right) position

Train stops

Main article: Train stop

The earliest systems were train stops, as still used by the New York City Subway, the Toronto subway, the London Underground, the Moscow Subway (only on the older lines) and the Berlin S-Bahn. Beside every signal is a moveable clamp, which touches a valve on a passing train. If the signal is red, the clamp opens the brake line, applying the emergency brake, If the signal shows green, the clamp is turned away.[1]

Trackside magnets for very simple data communication. Outside and middle of track: Integra-Signum, other two (yellow) magnets: ZUB

Inductive systems

In this system data is transmitted magnetically between the track and locomotive by magnets mounted beside the rails and on the locomotive.

In the Integra-Signum system the trains are influenced only at given locations, for instance whenever a train ignores a red signal, the emergency brakes are applied and the locomotive's motors are shut down. Additionally, they often require the driver to confirm distant signals (e.g. CAWS) that show stop or caution – failure to do so results in the train stopping. This gives sufficient braking distance for trains following each other, however it cannot always prevent accidents in stations where trains cross paths, because the distance from the red signal to the next obstacle may be too short for the train to brake to a halt.

More advanced systems (e.g., PZB, and ZUB) calculate a braking curve that determines if the train can stop before the next red signal, and if not they brake the train. They require that the train driver enter the weight and the type of brakes into the onboard computer. One disadvantage of this kind of system is that the train cannot speed up before the signal if the signal has switched to green, because the onboard computer's information can only be updated at the next magnet. To overcome that problem, some systems allow additional magnets to be placed between distant and home signals, or data transfer from the signalling system to the onboard computer is continuous (e.g., LZB).


Prior to the development of a standard train protection system in Europe, there were several incompatible systems in use. Locomotives that crossed national borders had to be equipped with multiple systems. In cases where this wasn't possible or practical, the locomotives themselves had to be changed. To overcome these problems, the European Train Control System standard was developed. It offers different levels of functionality, ranging from simple to complex. This model allows adopters to meet the cost and performance requirements of disparate solutions, from the smallest to the largest. The European system has been in operation since 2002 and uses GSM digital radio with continuous connectivity.

Cab signalling

Main article: Cab signalling

The newer systems use cab signalling, where the trains constantly receive information regarding their relative positions to other trains. The computer shows the driver how fast he may drive, instead of him relying on exterior signals. Systems of this kind are in common use in France, Germany and Japan, where the high speeds of the trains made it impossible for the train driver to read exterior signals, and distances between distant and home signals are too short for the train to brake.

These systems are usually far more than automatic train protection systems; not only do they prevent accidents, but also actively support the train driver. This goes as far as some systems being nearly able to drive the train automatically.


International standards

Country-specific systems

See also


  • Glover, John (1996). London's Underground. Ian Allan. ISBN 0-7110-2416-2. 


  1. Glover 1996, p. 91.
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