Signal passed at danger

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Two-aspect signal at danger

A signal passed at danger (SPAD), in British railway terminology, occurs when a train passes a stop signal without authority to do so.[1] It is a term primarily used within the British Railway Industry, although it can be applied worldwide.

Categories of SPAD

From December 2012,[2] the term "SPAD" only applies to category A SPADS. A new term, SPAR - Signal Passed at Red, will be used when describing category B, C and D incidents. There are a number of ways that a train can pass a signal at danger, and in the UK these fall into four basic categories:

  • A SPAD (Previously Category A SPAD) is where the train proceeds beyond its authorised movement to an unauthorised movement.[2]
  • A Technical SPAR (Previously Category B SPAD) is where the signal reverted to danger 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 Signaller SPAR (Previously Category C SPAD) is where the signal was replaced to danger in front of the train by the signaller in accordance with the rules and regulations and the train was unable to stop before passing the signal.
  • A Runaway SPAR (Previously Category D SPAD) is where an unattended train or vehicles not attached to a traction unit run away past a signal at danger.

Causes

As it takes a considerable distance to bring a train to a standstill, 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
  • Acute medical condition (medical emergency), such as a heart attack or stroke
  • Chronic medical condition, such as sleep apnea causing microsleep

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 they have 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

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

Driver's Reminder Appliance

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Main article: Driver reminder appliance

The DRA is an inhibiting switch designed specifically to prevent 'Starting Away SPADs' by passenger trains. The driver is required to operate the DRA switch whenever the train is brought to a stand, either[3]

  • after passing a signal displaying caution
  • or at a signal displaying danger

Once applied, the DRA displays a red light, and prevents Traction power from being taken.

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.

Train Detection

There are two main forms of train detection. Track circuits which detect the presence of trains and can, for example, hold signals at stop in the first place to prevent accidents. There is also axle counters which like track circuits, additionally count how many axles of a train has come in and out of the track in order to fully ensure no train is present. All other safety system such as train stops rely on detection systems such as track circuits and axle counters. Additionally a treadle is sometimes used.

Train stops

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Main article: Train stop

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 places where extensive tunnel operation is carried out, such as the Northern City Line where the Automatic warning system and Train Protection & Warning System are not fitted.

Train Protection & Warning System

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Main article: 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[4] may be used. Derailers and/or 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

SPAD indicator

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', some form of train detection, detects this and causes the SPAD indicator to flash red lights to warn the driver of the 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 faults may occur. They are designed to fail safe, so that when problems occur, the affected signal indicates danger (an example where this did not happen was the Clapham Junction rail crash due primarily to faulty wiring). To keep the network running, safety rules enable trains to pass signals that cannot be cleared to a proceed aspect. Provided that authority for the movement is obtained, a SPAD does not occur. Basically, there are two types of signal, and they are treated differently:

  • Automatic signals (those worked by the passage of trains) may be passed at danger by the driver under their own authority, but the driver must proceed slowly enough to be able to "stop short of any obstruction", eg another train ahead or a broken rail.
  • 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 they agree about how it is to be done. In the UK the signaller tells the driver of a specific train to pass a specific signal at danger, proceed with caution and travel at a speed that enables him to stop short of any obstruction, and then obey all other signals. If the signal is fitted with TPWS, the driver resets the Driver reminder appliance, pushes the TPWS Trainstop Override button in the cab, and proceeds cautiously through the section. If the train reaches the next signal without finding an obstruction, he must obey its aspect, at which point he can revert to normal working.

Accidents involving SPADs

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

Accidents involving stop and proceed

(In a stop and proceed accident, a train stops for, then passes, a stop signal according to the rules, but fails to keep to a low speed prepared to stop short of any obstruction)[citation needed]

See also

References

  1. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  2. ↑ 2.0 2.1 http://www.rgsonline.co.uk/Railway_Group_Standards/Traffic%20Operation%20and%20Management/Railway%20Group%20Standards/GORT3119%20Iss%203.pdf
  3. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  4. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  5. ↑ Lua error in package.lua at line 80: module 'strict' not found.
  6. ↑ Lua error in package.lua at line 80: module 'strict' not found.

External links

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