Rail Engineer • June 2015
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drivers to correctly correlate AWS warnings in the cab with signals or speed restriction indicators on the line ahead. In particular, such portable AWS magnets shall not be positioned between any other AWS equipment and its associated signal, board or indicator.
AWS Achilles’ Heels After receiving and acknowledging a warning, the safety of the train is entirely in the hands of the driver in comprehending the warning and driving appropriately in accordance with signals and speed limits. It was never foreseen that a driver, having acknowledged the warnings leading up to a red signal, would then just drive on. Secondly, if a station stop intervenes between an AWS warning and the signal at red, there is the risk that once station duties have been completed, the guard gives ‘ding-ding’ and the train starts away but the driver fails to check whether the signal is still red. Also, it is possible to isolate the AWS and drive without protection as happened with an HST at Southall in 1997 (seven killed, 139 injured).
Train Protection Warning System (TPWS) Acknowledging the weaknesses of AWS, BR commenced work in 1988 on a more effective system which became known as the Train Protection Warning System (TPWS). It was mandated by the 1999 Railway Safety Regulations. Fitment was completed in 2003 with suppliers including Thales, Redifon, and Unipart Rail. Sadly, whilst the project was being planned, a further disastrous SPAD occurred at Ladbroke Grove in 1999 (31 killed, 227 hospitalised).
TPWS is designed to stop a train in three situations. At selected signals a train stop (TSS) will be provided at the signal and apply the brakes in the event of a SPAD. At selected signals an overspeed (OSS) facility will operate and apply the brakes should a train approach a red signal too fast. TPWS is designed to stop a train before reaching a point of conflict. Additional overspeed sensors may be provided (TPWS+) to cope with higher speeds. At other locations, such as on the approach to a permanent speed restriction or buffer stop, the OSS will apply the brakes in case of excessive speed. A weakness of TPWS is that the driver may ‘reset and continue’. Like AWS, TPWS may also be ‘isolated’ thereby removing the ability to protect the train. Furthermore, TPWS is not fitted at every signal as the primary objective is to protect junctions. The standard TPWS driving cab display unit delivers a flashing visual indication for a brake demand, leaving the driver to work out whether the brake application was due to SPAD, Overspeed or AWS, leading to possible confusion and belief that the equipment is faulty. If the TPWS system initiates a brake application, the TPWS brake demand indicator will flash. There will be no audible warning. Once the AWS/ TPWS acknowledgement button is pressed and released, the TPWS brake demand indicator will go steady. The brakes will release and the indicator will clear 60 seconds after the brake demand was initiated. An enhanced TPWS control panel has recently been developed with three indicator buttons ‘SPAD’, ‘Overspeed’ and ‘AWS’. Whenever a
brake demand is initiated because of a SPAD or overspeed, the respective flashing indicator is accompanied by a spoken message, preceded by an ‘alert’ tone. This says ‘SPAD alert, contact signaller’ or ‘Overspeed, contact signaller’ as appropriate and continues until the driver presses the appropriate indicator button.
In the signal box Signallers were criticised for not responding quickly enough when they became aware of the SPAD at Ladbroke Grove but it is unrealistic to expect a signaller to continuously monitor the progress of every train moving within their sphere of control, particularly in a busy box with the intensive operational activity that is in progress and ever changing. The signaller may, for example, be engrossed in conversation with a depot about an impending freight movement, or have momentarily left the workstation to read a notice about an emergency speed restriction that has just printed out. If a SPAD takes place, a signaller needs to become immediately aware of the potential seriousness of the situation and make a split second decision as to the best course of action to stand any chance of avoiding a collision. The options are limited and may include sending a STOP radio message to driver/s in the locality, putting signals back to danger, and trying to divert the errant or other trains away from possible conflict although the latter may be prevented by the SPAD train occupying train detection sections and deadlocking points that could otherwise divert the train away from danger.