The driver in the Southall rail crash, where seven people died, would normally have had two warnings that he was approaching a red signal, but in that case the AWS was not working. The situation in the case of the Paddington crash is not entirely clear, but it is thought that a faulty track could have triggered the wrong noise in the cabin of the driver of the Thames train leading him to believe he had been given a green signal instead of a red one. It was revealed at the inquiry that Michael Stodder, the driver of the Thames Train, had not received training in red signal awareness. The Health and Safety Executive has said that TPWS would have prevented the west London crash. So far Railtrack and the government have invested 140m in the system.
Computerised equipment is installed in both the train and at key points along the track.
The system is designed to stop trains travelling at up to 200mph from going through danger signals. In addition, monitoring devices between tracks register the train's speed and activate its brakes if it is going too fast. Ironically, the stretch of line on which the Paddington crash happened is one of the few in the country where ATP is used. The Great Western train was fitted with it, which, if it had been working, would have prevented the train from running through a danger signal. The Thames turbo unit did not have ATP although it was being tested on the lines out of Paddington.