Signal passed at danger

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This term is often abbreviated to SPAD. For other uses of the acronym, see SPAD (disambiguation).

Two-aspect signal 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.

1 Causes
2 Prevention
- 2.1 Train stops
- 2.2 Automatic Train Protection
3 SPAD indicators
4 Stop and proceed
5 Accidents involving SPADs
6 References
7 External links

[edit] Causes

Most SPADs occur at low speed where braking distance has been misjudged, and the train can subsequently be stopped within a safety overlap area, 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 crash at a time when TPWS was not in operation.

Reasons why engine 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.

[edit] Prevention

[edit] Train stops

Main article: Train stop

On the 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.

[edit] Automatic Train Protection

Main article: 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.

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[edit] SPAD indicators

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 circuit or 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.

[edit] 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 disaster can 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.