During the 1980s, an increase in trains passing signals at danger caused British Rail to develop a more comprehensive system than the existing Automatic Warning System. In 1988, a three-year program was launched to develop a system available to be implemented by 1992. Unlike the existing AWS system, which only advised the driver, leaving room for human error, Automatic Train Protection would be able to take control of the train, ensuring it was driven according to the conditions. The two pilot schemes for the project were London Paddington to Bristol, and London Marylebone to High Wycombe and Aylesbury. As a pilot scheme, the two systems were slightly different, with the Western scheme using ACEC equipment, and Chiltern using GEC General Signal. There were plans to expand the system more widely, with BR stating that ATP would be installed on "a large percentage of its network". Railtrack, who took over the management of rail infrastructure in 1994, later committed only to completion of the two pilot schemes, adaptation for new high speed lines, and to search for a cheaper alternative for the rest of the network. It was later decided that TPWS would be rolled out instead of ATP. This system automatically stops trains that pass red signals or speed restrictions at too high a speed, but does not monitor speed constantly, hence only mitigating SPADs, rather than preventing them. The total cost of ATP was estimated to be £750 million, equivalent to £1.131 billion in 2020. In 1994 British Rail and Railtrack estimated that this would come to £14 million per life saved, compared to the £4 million per life they considered good value for money. A Health and Safety Commission report later estimated a full installation of ATP would cost £11 million per life saved, or £5 million if only installed at high risk locations, figures Railtrack agreed with.
Function
The aim of ATP was to prevent trains from both exceeding speed restrictions, and from passing signals at danger. The system was beacon based, with information transmitted to the train at fixed beacons. The on board computer takes track and signal information from the beacons, and calculates the maximum speed of the train. When the maximum permitted speed decreases, such as when approaching a signal at danger, three braking curves are calculated: the indication curve, which is the ideal deceleration to the new limit; the warning curve, above the indication curve, which causes a warning to the driver; and the intervention curve, above the indication curve. At this point, the train will apply the brakes automatically.
