Automatic Train Protection

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Automatic Train Protection (ATP) in Great Britain refers to either of two implementations of automatic train protection system installed in some trains in order to help prevent collisions through a driver's failure to observe a signal or speed restriction. Note that ATP can also refer to automatic train protection systems in general, as implemented in other parts of Europe and elsewhere.

This system uses a target speed indication and audible warnings to warn the train driver if they are likely to exceed a speed profile that will cause the train to pass a red signal or exceed a speed restriction. The system will apply the brakes if the driver fails to respond to these warnings. The system takes into account the speed and position of the train relative to the end of its 'movement authority' in issuing the warnings and applying the brakes.

By the 1980s microprocessors had developed sufficiently for BR to carry out pilot trials on existing European ‘off the shelf’ ATP –fitting part of the Great Western Main line with the TBL1 system from ACEC and the Chiltern Main Line route with SELCAB a derivative of the German LZB system from Alcatel and GEC.

In the early 1990s, following the Clapham Junction rail crash in December 1988, and two other fatal accidents in early 1989 caused by SPADs, British Rail was keen to implement the ATP system across the entire British railway system. However, the cost (estimated at over £1bn) was baulked at by the Conservative government, who were preparing the company for privatisation.

All First Great Western's High Speed Trains (HSTs) are now fitted with ATP, and are not allowed to carry passengers unless the system is functioning. This requirement is in response to the Ladbroke Grove rail crash.

ATP is given permitted speed and location information from the track via encoded balise(s), encoded track circuit or more recently via radio.