Train Protection & Warning System
The Train Protection & Warning System (TPWS) is a train protection system deployed across the entire UK passenger railway network, as well as in Victoria, Australia. It automatically activates brakes on any train that has passed a signal at danger or is overspeeding. It is fitted at selected sites, including some lines where automatic train protection (ATP) is installed.
Unlike ATP, it does not aim to stop trains at or before a signal that is at "danger" - it aims to stop the train before the point at which a collision with another train could occur, excluding rear-end collision with a train in front. According to the UK's Health and Safety Executive, TPWS is estimated to prevent between 65% and 80% of fatalities that would be stopped by a full ATP system.
A standard installation consists of an on-track transmitter placed adjacent to a signal and activated when the signal is at 'danger'. Any train that tries to pass the signal will have its emergency brakes activated. If the train is travelling at speed, this may be too late to stop it before the point of collision, therefore a second transmitter may be placed on the approach to the signal that applies the brakes on trains going too quickly to stop at the signal, and this is positioned to safely stop trains approaching at up to 75 mph (120 km/h).
At around 400 high-risk locations, a third transmitter is used even further before the signal and this increases the effectiveness to 100 mph (160 km/h). An installation with three transmitters is known as TPWS+. When installed in conjunction with signal controls such as 'double blocking' (i.e. two red signal aspects in succession), TPWS can be fully effective at any realistic speed.
TPWS is not to be confused with timed train stops that accomplish a similar task with different technology.
Read more about Train Protection & Warning System: Limitations, In Use By
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