- Signal passed at danger
Signal passed at danger (SPAD), in
railway terminology, describes an event where a train has run beyond its allocated signal block without authority, as indicated by a lineside signal showing danger (typically a red light). It is a term primarily used within the British Railway Industry, although it can be applied worldwide.
Most SPADs occur at low speed where
braking distancehas been misjudged, and the train can subsequently be stopped within a safety overlaparea, either by the engineer/driver or by an automatic application of the emergency brakes by a safety system, such as the Train Protection & Warning System(TPWS). In some situations where a signal cannot be seen due to obstruction, sufficient braking may not have been achieved, and the train will continue to run a significant distance beyond the danger signal. Such an incident was determined as the main cause of the Ladbroke Grove rail crashat a time when TPWS was not in operation.
Reasons that drivers do not see red signals include:
fog, falling snow, badly designed signal layouts, inattention and being distracted, signal light dim or dirty or lamp failure.
Almost all railways require a dim or dark signal to be treated as if it were displaying its most restrictive aspect, i.e. 'stop' for a stop signal or 'caution' for a distant signal. In most cases, the type of signal can be determined by a plate or coloured marking on the signal post. A somewhat related fault is when a semaphore signal's arm is stuck in the 'clear' position, e.g. by frost or snow.
London Underground(for example), train stops are fitted on the track to stop a train, should a driver cause a SPAD. Should a train be stopped under such circumstances, delays occur because the train's trip cock has to be reset, and a replacement driver found as the offending driver is not permitted to continue with the train. Train stops (and trip cock equipped trains) are also operated by the main line railways, in many places where extensive tunnel operation is carried out.
Automatic Train Protection
ATP (Automatic Train Protection) is a much more advanced form of Train Stop, which can regulate the speed of trains in many more situations other than at a stop signal. ATP supervises speed restrictions and distance to danger points. An ATP does take into account individual train characteristics such as brake performance. Thus, the ATP determines when brakes should be applied in order to stop the train before getting beyond the danger point.
Prior to the introduction of
Train Protection & Warning System(TPWS) in the UK, "SPAD indicators" were introduced at 'high risk' locations (for example: the entry to a single track section of line). These SPAD indicators are placed beyond the protecting stop signal and are normally unlit. Should a driver pass the signal at 'danger', a track circuitor treadle detects this and causes the SPAD indicator to flash red lights to warn the driver of his error. Since the introduction of TPWS, provision of new SPAD indicators has become less common.
Stop and proceed
Automatic signals may be passed at 'danger' in accordance with the rules. The driver must proceed with extreme care as there may be obstacles in the way. If the driver out of habit forgets to maintain a low speed, accidents such as the
Glenbrook train disastercan occur.
Signals passed under these rules are not classified as SPADs. In general, this can only be done under the direction of a signalman, but rules vary greatly between countries and even systems with countries.
Accidents involving SPADs
* - Ais Gill disaster, 1913 (UK)
* - Potters Bar, 1946 (UK)
Harrow and Wealdstone rail crash, 1952 (UK)
Lewisham rail crash, 1957 (UK)
Dagenham East rail crash, 1958 (UK)
Harmelen train disaster, 1962 (Netherlands)
* - Violet Town, Victoria, 1969 (Australia)
Glasgow Bellgrove rail crash, 1989 (UK)
Purley station rail crash, 1989 (UK)
* - Newton rail crash, 1991 (UK) - also single lead junction
Norton Bridge rail crash, 1991 (UK)
Cowden rail crash, 1994 (UK)
Southall rail crash, 1997 (UK)
* - Beresfield, New South Wales, 1997 (Australia)
Winsford rail crash, 1999 (UK)
Ladbroke Grove rail crash, 1999 (UK) - a SPAD that led to dozens of deaths. Prompts TPWS.
* - Pécrot Rail Crash, 2001 (BE)
Arnhem rail crash- 2006 (Netherlands)
2008 Chatsworth train collision(US) - Based on preliminary report by railway -- official cause pending.
* [http://www.hse.gov.uk/railways/spads.htm UK Health and Safety Exec] , Retrieved
8 March 2006.
* [http://www.rail-reg.gov.uk/server/show/ConWebDoc.7788 Office of Rail Regulation]
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