Train Protection & Warning System
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The Train Protection & Warning System (TPWS) has been deployed across the entire UK passenger railway network.[1] Its purpose is to safely stop trains that pass signals at danger so as to avoid a collision. It is used across the country, including places where automatic train protection (ATP) is installed.
Unlike ATP, it does not aim to stop trains at or before a signal that is set to 'danger'. It aims to stop the train in the overlap area beyond the signal. The overlap is the margin of safety that is always left between trains. 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.
TPWS is an interim measure until a full ATP system is installed across the country. Standard TPWS is fully effective up to a speed of 75mph (120km/h). An uprated version of TPWS called TPWS+ is generally effective up to 100mph (160km/h) and has been deployed in around 400 high risk locations around the country. 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 speed.
TPWS is not to be confused with timed train stops that accomplish a similar task with different technology.
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[edit] How it works
[edit] Overview
One pair of electronic loops is placed 50-450 metres on the approach side of the stop signal and they are energised when this signal is at 'danger'. The distance separating the loops is used to control the speed of the train, because at all locations the on-board equipment allows a standard amount of time between paired loops before applying the brakes: the greater the distance the loops are from the stop signal, the more widely spaced they will be.
There is another pair of loops at the stop signal, also energised when the signal is at 'danger'. These are always placed immediately together and will stop a train that runs past the signal regardless of its speed.
[edit] On Track Equipment
For a "standard" installation of TPWS, there are four loops used. Both pairs consist of an 'arming' and a 'trigger' loop. If the signal associated with the TPWS is at 'danger', the loops will be energised. If the signal is at 'proceed', the loops will de-energise.
The first pair, the Overspeed Sensor System (OSS), is sited at a position determined by linespeed and gradient. The loops are set apart by a distance that should not be traversed within a pre-determined period of time (approximately 1 second) as long as the train is running at a safe speed approaching the signal at 'danger'.
The first loop is known as the 'arming loop', and emits a frequency of 64.25kHz. The second loop is called the 'trigger loop', and its frequency is 65.25kHz.
The other set of loops is positioned back to back at a signal, and are called a Train Stop System (TSS). The TSS also comprises 'arming' and 'trigger' loops, which in this case work at 66.25kHz and 65.25kHz, respectively. The brakes will be applied if the on-train equipment detects both frequencies together after having detected the arming frequency alone. Thus, an energised TSS is effective at any speed, but only if a train passes it in the applicable direction. Since a train may be required to pass a signal at danger during failure etc., a driver has the facility to override a TSS, but not an OSS.
For opposite direction TPWS equipment, the frequencies are slightly different, working at 64.75, 65.75, and 66.75kHz.
[edit] Location Equipment
In the lineside location, there are two modules associated with each set of TPWS loops. There will be a Signal Interface Module (SIM) and an OSS or TSS module. These generate the frequencies to the loops, and prove the loops are intact. They also interface with the legacy signalling system.
[edit] On Train Equipment
An aerial on the train picks up the frequency from the loops if they are energised, and applies the brakes if required, for instance, if it takes less than 1 second (approximate value) to travel over the OSS loops, or if the TSS loops are energised. When the train passes over the first pair of loops, a timer counts the amount of time between the loops. This time is used to test the speed and if the train is approaching too fast, the brakes are applied to stop it within the overlap. If the train passes the first test but passes the signal at 'danger', the automatic brakes will be applied and stop it in the overlap.
[edit] Variations
In some instances, due to low linespeeds, an OSS may not be fitted to a signal. An OSS on its own may be used to protect a permanent speed restriction or buffer stop. Although loops are standard, buffer stops may be fitted with 'mini loops', due to the very low approach speed.
Recent applications in the UK have, in conjunction with advanced SPAD protection techniques, used TPWS with 'outer home' signals that protect junctions with a higher than average risk, by controlling the speed of an approaching train an extra signal section in rear of the junction. If this fails for any reason then the resultant TPWS application of brakes will stop the train before the actual point of conflict is reached.
[edit] Limitations
TPWS has no ability to regulate speed after a train passes a signal at stop in accordance with Stop and Proceed rules.
Thus TPWS would not help prevent the following accidents:
[edit] In use by
The TPWS system is used in
- Great Britain and Northern Ireland with AWS magnets, and short overlaps
- Victoria, Australia albeit without any AWS magnets, but with full length overlaps.
[edit] List of accidents preventable by TPWS
- Ladbroke Grove rail crash of 1999, 31 killed
- Purley Station rail crash of 1989, 5 killed
- Invergowrie rail crash of 1979, 5 killed
