Train Protection & Warning System

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. [ [ Newtork Rail: STEP CHANGE IN SAFETY DELIVERED ON TIME AND UNDER BUDGET - Monday 29 December 2003] ] Consisting of electromagnetic loops on the track and corresponding receivers on the train, it automatically activates brakes on any train that has passed a signal at danger or is overspeeding. It is fitted at selected sites, including 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.

TPWS is an interim measure until a full ATP system is installed. Standard TPWS is generally fully effective up to a speed of 75 mph (120 km/h), and an uprated version called TPWS+ is generally effective up to 100 mph (160 km/h) and has been deployed at around 400 high-risk locations. 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.

How it works


One pair of electronic loops is placed 50-450 metres on the approach side of the stop signal, 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 signal, the more widely spaced they will be.

There is another pair of loops at the 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.

On-track equipment

In a standard installation of TPWS there are two pairs of loops. 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 separated by a distance that should not be traversed within a pre-determined period of time (approximately 1 second) if the train is running at a safe speed approaching the signal at "danger".

The first, 'arming', loop emits a frequency of 64.25 kHz. The second, 'trigger', loop has a frequency of 65.25 kHz.

The other pair of loops is back to back at the signal, and is called a Train Stop System (TSS). The 'arming' and 'trigger' loops work at 66.25 kHz and 65.25 kHz 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., the 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 66.75 kHz.

Location equipment

In the lineside location, there are two modules associated with each set of TPWS loops: a Signal Interface Module (SIM) and an OSS or TSS module. These generate the frequencies for the loops, and prove the loops are intact. They interface with the legacy signalling system.

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 (e.g. 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 OSS loops, a timer counts the amount of time between the loops. This time is used to check 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.


In some instances, due to low linespeeds, an OSS may not be fitted.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 the resultant TPWS application of brakes will stop the train before the actual point of conflict is reached.


TPWS has no ability to regulate speed after a train passes a signal at "stop" in accordance with Stop and Proceed rules. However there are strict rules governing drivers' actions and train speed when passing signals at "danger" with authority, and the use of TPWS on these occasions.

Thus TPWS would not have helped prevent the Glenbrook train disaster.

In use by

The TPWS system is used in
* The United Kingdom, with AWS magnets and with short overlaps
* Victoria, Australia, without AWS magnets and with full-length overlaps.

List of accidents preventable by TPWS

* Ladbroke Grove, 1999, 31 killed
* Purley, 1989, 5 killed
* Invergowrie, 1979, 5 killed


External links

* [ Thales Communications. Manufacturers of TPWS]
* [ Office of Rail Regulation (replaces HSE Rail division)]

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