Signal passed at danger

Two-aspect signal at danger

Signal passed at danger (SPAD), in British 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.

Categories of SPAD

There are a number of ways that a train can pass a signal at Danger without authority, and in the UK these fall into 4 basic categories;

• A Category A SPAD is where the signalling equipment was working properly, but the train passed the red signal aspect even though there was adequate opportunity for it to stop
• A Category B SPAD is where the signal reverted to a red aspect in front of the train due to an equipment failure or signaller error, and the train was unable to stop before passing the signal.
• A Category C SPAD is where the signal reverted to a red aspect in front of the train due to an emergency, and the train was unable to stop before passing the signal.
• A Category D SPAD is where the train ran out of control and could not be stopped from passing a red aspect signal.

Causes

Because it takes a considerable distance to bring a train to a stand, many Category A SPADs occur at low speed where the driver has applied the brakes too late. Very often this occurs when the signal at danger cannot be clearly discerned until close up. It can also be due to;

• Inattention
• Distraction
• Fatigue
• Misreading of an adjacent signal due to line curvature, or sighting on one beyond
• Misunderstanding
• Miscommunication with a Signaller or Shunter
• Signal being poorly lit or mis-showing

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.

In some situations however the driver is unaware that he has passed a signal at danger and so continues until a collision occurs, as in the Ladbroke Grove rail crash. In this instance it is up to the safety system (where fitted) to apply the brakes, or for the signaller to alert the driver.

Prevention

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. In the UK, only a small percentage of trains (First Great Western and Chiltern Trains) are fitted with this equipment.

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Collision avoidance

Whilst the ideal safety system would prevent a SPAD from occurring, most equipment in current use does not stop the train before it has passed the Danger signal. However, provided that the train stops within the designated overlap beyond that signal, a collision should not occur.

Track circuits

Track circuits which detect the presence of trains and can for example can hold signals at stop in the first place prevent many accidents. All other safety system such as train stops rely on detection systems such as track circuits.

Train stops

On the London Underground (for example), train stops are fitted on the track to stop a train, should a SPAD occur. When a train is stopped under such circumstances, delays occur because the train's trip cock has to be reset, and a replacement needs to be found as the 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.

Train Protection & Warning System

On the UK mainline, TPWS consists of an on-board receiver/timer connected to the emergency braking system of a train, and radio frequency transmitter loops located on the track. The 'Overspeed Sensor System' pair of loops is located on the approach to the signal, and will trigger the train brakes if it approaches faster than the 'set speed' when the signal is at danger. The 'Train Stop System' pair of loops is located at the signal, and will trigger the brakes if the train passes over them at any speed when the signal is at danger.

TPWS has proved to be an effective system in the UK, and has prevented several significant collisions. However its deployment is not universal; only those signals where the risk of collision is considered to be significant are fitted with it.

Flank protection

At certain junctions, especially where if the signal protecting the junction was passed at danger a side collision is likely to result, then flank protection may be used. Facing points beyond the signal protecting the junction will be set in such a position to allow a safe overlap if the signal was passed without authority. This effectively removes the chance of a side-impact collision as the train would be diverted in a parallel path to the approaching train.

SPAD indicators

Prior to the introduction of 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 circuit or treadle detects this and causes the SPAD indicator to flash red lights to warn the driver of his error. Whenever a SPAD indicator activates, all drivers who observe it are required to stop immediately, even if they can see that the signal pertaining to their own train is showing a proceed aspect. Since the introduction of TPWS, provision of new SPAD indicators has become less common.

Passing signals at danger - with authority

Signals form part of a complex system, and it is inevitable that there will be times when they go wrong. They are designed to Fail Safe, so that when problems do occur the affected signal will revert to danger (one notable exception to this being the Clapham Junction rail crash). In order to keep the network running there are safety rules which enable trains to pass signals which can't be cleared to a proceed aspect, and provided that authority for the movement is obtained then a SPAD will not have occurred. Basically there are two types of signal, and they are treated differently;

• Automatic signals (defined as those which are worked by the passage of trains) may be passed at danger by the driver under his own authority (however this is normally authorised by communicating with the controlling signaller). If the driver is stood at an automatic signal at danger and is unable to contact the signaller by all possible means, he is entitled to pass that signal at danger. He must proceed with extreme care as he will be expecting to find a train or other obstruction, such as a broken rail, in that section. If the driver fails to regulate the speed of his train, and is unable to stop it within the distance which he can see to be clear, accidents such as the Glenbrook train disaster can occur. In any case as soon as he reaches the next signal, he must stop and inform the signaller what he has done - even if that signal is showing a proceed aspect.

• Controlled signals (those worked by the signaller to control the entry to junctions or conflicting movements) can only be passed at danger with the signaller's authority. The driver and signaller must both come to a clear understanding and ensure that they are in agreement about how it is to be done. In the UK the signaller will tell the driver of a specific train to pass a specific signal at danger, proceed with caution and travel at a speed which enables him to stop short of any obstruction, and to then obey all other signals. If the signal is fitted with TPWS he will advise the driver of this. Then if necessary the driver will push the TPWS Trainstop Override button in his cab, sound his horn and proceed cautiously through the section. If he reaches the next signal without having found any obstruction, he will obey its aspect, at which point he will revert to normal working.

Accidents involving SPADs

This list is incomplete; you can help by expanding it.

• - Slough, 1900 (UK)
• - Tonbridge, 1909 (UK)
• - Ais Gill, 1913 (UK)
• - Charfield, 1928 (UK)
• - Norton Fitzwarren, 1940 (UK)
• - Eccles, 1941 (UK)
• - Potters Bar, 1946 (UK)
• - Harrow and Wealdstone, 1952 (UK)
• - Lewisham, 1957 (UK)
• - Dagenham East, 1958 (UK)
• - Harmelen, 1962 (Netherlands)
• - Marden, 1969 (UK)
• - Violet Town, Victoria, 1969 (Australia)
• - Paisley Gilmour Street, 1979 (UK)
• - Invergowrie, 1979 (UK)
• - Wembly Central, 1984 (UK)
• - Eccles, 1984 (UK)
• - Hinton, 1986 (Canada)
• - Colwich Junction, 1986 (UK)
• - Chase, MD, 1987, (US)
• - Glasgow Bellgrove, 1989 (UK)
• - Purley, 1989 (UK)
• - Shigaraki, 1991 (Japan)
• - Newton, 1991 (UK) - also single lead junction
• - Cowden, 1994 (UK)
• - Silver Spring, MD, 1996 (US)
• - Hines Hill, 1996 (Australia)
• - Southall, 1997 (UK)
• - Beresfield, New South Wales, 1997 (Australia)
• - Spa Road Junction, 1999 (UK)
• - Winsford, 1999 (UK)
• - Ladbroke Grove, 1999 (UK) - a SPAD that led to dozens of deaths. Prompts TPWS.
• - Pécrot, 2001 (Belgium)
• - Norton Bridge, 2003 (UK)
• - Qalyoub, 2006 (Egypt)
• - Arnhem, 2006 (Netherlands)
• - Chatsworth, CA, 2008 (USA) - Based on preliminary report by railway -- official cause pending.

Accidents involving Stop and proceed

(In a stop and proceed accident, a train passes a stop sign according to the rules, but fails to keep to a low speed prepared to stop short of any obstruction)

• - Lindfield, 1928 (Australia)
• - Stratford (London Underground), 1953 (UK)
• - Coppenhall Junction, 1962 (UK)
• - Wrawby Junction, 1983 (UK)
• - Glenbrook, 1999 (Australia)
• - Vittorio Emanuele (Rome Metro), 2006 (Italy)

References

• UK Health and Safety Exec, Retrieved 8 March 2006.

External links

• Office of Rail Regulation