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The Train Protection & Warning System (TPWS) is a train protection system deployed across the entire UKmarker passenger railway network, as well as in Victoriamarker, 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 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.

How it works

Overview

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

A TPWS transmitter loop, one of a pair that form an Overspeed Sensor System (OSS)


In a standard installation of TPWS there are two pairs of loops (sometimes colloquially referred to as "grids" or "toast racks"). 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-cab equipment

TPWS panel in driving cab


The TPWS panel is located in the train cab along with a TPWS temporary isolation switch. The TPWS panel comprises two indicator lamps and a push switch. One lamp is used to indicate that a TPWS/AWS brake demand has occurred (the AWS and TPWS system are inter-linked). A temporary isolation indicator/fault indicator indicates that the system has been isolated with the temporary isolation switch or a fault has occurred with the TPWS. The push switch marked "train stop override" is used to pass a signal at danger with authority; it temporarily will ignore the TPWS TSS loops at a signal for a period of around 20 seconds or as soon as the loops have been passed.

The temporary isolation switch may be operated when the train is being operated in degraded conditions and multiple stop aspects will be required to be passed at danger with authority, after which the system must immediately be reinstated.

Variations

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, usually 10 mph. When buffer stops were originally fitted with TPWS using standard loops, there were many instances of false applications, causing delays whilst it reset (with trains potentially blocking the station throat) plus the risk of passengers standing to alight being thrown over by the sudden braking. This problem arose when a train passed over the arming loop so slowly that it was still detected by the train's receiver after the on-board timer had completed its cycle. The timer would reset and begin timing again, and the trigger loop then being detected within this second timing cycle would lead to a false intervention. As a temporary solution, drivers were instructed to pass the buffer stop OSSs at 5 mph, eliminating the problem, but meaning that trains no longer had the momentum to roll to the normal stopping point and requiring drivers to apply power beyond the OSS, just a short distance from the buffers, arguably making a buffer stop collision more likely than before TPWS was fitted. The redesigned 'mini loops', roughly a third the length of the standard ones, eliminate this problem, although due to the low speed and low margin, buffer stop OSSs are still a major cause of TPWS trips.

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. This system is referred to as TPWS OS (Outer Signal).

Limitations

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.

There are several reasons why a driver would be required to pass a signal at danger with authority, such as track circuit failure. The driver will have been advised by the signaller to pass the signal at danger, proceeding with caution and being prepared to stop short of any obstruction, and to obey all other signals. The signaller would expect the driver to know the requirement of pressing the "Trainstop Override" button on the TPWS panel to pass the signal without triggering a TPWS brake demand. A signaller is not allowed to inform the driver to operate any TPWS override, only that TPWS may be present at the signal that is required to be passed at Danger.

Due to the varying conditions under which a train may need to pass a signal at danger, it is seen as best to leave the appropriate speed to the driver's discretion, rather than have a fixed speed. The appropriate speed for a heavy freight train on a curved line during heavy rain will be much lower than for a fast braking passenger train on a straight line in clear and dry conditions.

Whilst critics claim TPWS is a cheap solution, and putting lives at risk compared to fitting ATP, there have been very few fatalities in modern times (since the fitting of AWS) that would have been prevented had ATP been fitted but would still have occurred despite TPWS. The Southall rail crashmarker would not have been prevented by TPWS yet could have been prevented by ATP (ironically fitted but not in use), yet would almost certainly have been prevented had the AWS been working. A combination of TPWS and AWS is most weak against accidents like that at Purley, where a driver repeatedly cancelled the AWS warning without applying the brakes, passing the danger signal at high speed. In this particular case though, the lower speed of the train and the very effective brakes of the EMU would have meant TPWS would likely have been effective anyway. Supporters of TPWS claim that even where it could not prevent accidents due to SPADs, it would likely reduce the impact and reduce or eliminate fatalities by at least slowing the train down, but in practice it is likely that in these cases the driver would have already applied the emergency brakes well before the overspeed sensor.

In use by

The TPWS system is used in

List of accidents preventable by TPWS



References

  1. Newtork Rail: Step change in safety deliverd on time and under budget - Monday 29 December 2003


External links




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