Asignal passed at danger (SPAD)^[1]: 75 is an event on arailway where a train passes a stopsignal without authority.^[2] This is also known asrunning a red, in theUnited States as astop signal overrun (SSO)^[3] and in Canada aspassing a stop signal.^[4] SPAD is defined byDirective 2014/88/EU as any occasion when any part of a train proceeds beyond its authorised movement.^[5] Unauthorised movement means to pass:^[5]

• a trackside colour light signal or semaphore at danger, or an order to STOP where aTrain Protection system (TPS) is not operational,
• the end of a safety related movement authority provided in a TPS,
• a point communicated by verbal or written authorisation laid down in regulations,
• stop boards (buffer stops are not included) or hand signals.

The name derives from redcolour light signals and horizontalsemaphore signals in the United Kingdom, which are said to beat danger when they indicate that trains must stop (also known as the signal beingon). This terminology is not used in North America where not all red signals indicate stop.^[1]: 72 In the UK, asignal passed at red (SPAR) is used where a signal changes to red directly in front of a train, due to a fault or emergency, meaning it is impossible to stop before the signal.

The highinertia of trains, and the lowadhesion between the wheels and track, means it takes a longdistance for thetrain brakes to stop a train. SPADs are most commonly a small overrun of the signal (instead of a long overrun), because the driver has braked too late. The safety consequences for these types of SPADs may be minor. On the other hand, some SPADs involve the driver being unaware they have passed a signal at danger and continue until notified by network controllers, or a collision occurs, as in theLadbroke Grove rail crash.

The causes and prevention of SPADs is actively researched. Causes of SPADs are always multidimensional.^{[citation needed]} Some of the causes of SPADs are:

• Misjudgement^[6]
• Environmental conditions
• Inattention
• Distraction^[4]
• Fatigue^[7][8]
• Misreading of an adjacent signal due to line curvature (a “crossread”), or sighting on one beyond^[6]
• Misunderstanding
• Miscommunication^[6]
• Incomplete or lapsedroute knowledge
• Acute medical condition (medical emergency), such as aheart attack orstroke
• Chronic medical condition, such assleep apnea causingmicrosleep
• Driver deviation from standard operating procedures^[9]
• False assumptions based on familiarity and past situations^[9]
• Poor change management practices^[9]
• Failure to properly assess competencies^[9]

Automatic train protection (ATP) is an advanced form of train stop which can regulate the speed of trains in situations other than at a signal set at danger. ATP can supervise speed restrictions and distance to danger points. It can also take into account individual train characteristics such as brake performance etc. Therefore ATP can determine when brakes should be applied in order to stop the train before passing a signal at danger. Presently, In the UK, only a small percentage of trains (Great Western Railway andChiltern Railways) are fitted with this equipment.

Thedriver's reminder appliance (DRA) is an inhibiting switch located on the driver's desk of United Kingdom passenger trains designed specifically to prevent "starting away SPADs". The driver is required to operate the DRA whenever the train is brought to a stand,^[10] either 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 until the DRA is manually cancelled by the driver.

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 designatedoverlap beyond that signal, a collision should not occur.

On theLondon Underground (for example), mechanicaltrain stops are fitted beside the track at signals to stop a train, should a SPAD occur.

Train stops are also installed on main line railways in places where tripcock equipped trains run in extensive tunnels, such as the on theNorthern City Line where theAutomatic warning system andTrain Protection & Warning System are not fitted.

On the UK mainline,AWS consists of an on-board receiver/timer connected to theemergency braking system of a train, and magnets located in the center of the track. At each AWS site, a permanent magnet arms the system and an electromagnet connected to the green signal lamp disarms the system and a confirming chime is provided to the driver. If the receiver does not disarm within one second after arming, a warning tone sounds at the driver's desk and if it is not cancelled by the driver, the emergency brakes will be activated. A visual indication remains set to remind the driver that they have passed a restrictivesignal aspect.

On the UK mainline,TPWS consists of an on-board receiver/timer connected to theemergency 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 activate the train's emergency brake if it approaches faster than the 'trigger speed' when the signal is atdanger. The 'Train Stop System' pair of loops is located at the signal, and will activate the emergency brake if the train passes over them at any speed when the signal is atdanger.

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

At certain junctions, especially where if the signal protecting the junction was passed atdanger a side collision is likely to result, then flank protection^[11] 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.

Prior to the introduction ofTPWS in the UK, "SPAD indicators" were introduced at 'high risk' locations (for example: the entry to a single track section of line). Consisting of three red lamps, they are placed beyond the protecting stop signal and are normally unlit. If a driver passes the signal at 'danger', the top and bottom lamps flash red and the centre lamp, which has the word "STOP" written across the lens in black, is lit continuously. 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 aproceed aspect. Since the introduction of TPWS, provision of new SPAD indicators has become less common.

In the UK, incidents where a signal is passed at danger without authority are categorised according to principal cause. ASPAD is where the train proceeds beyond its authorised movement to an unauthorised movement. Other types are categorised asSPAR ("signal passed at red").

Prior to December 2012,^[12] the term "SPAD" applied to all such incidents, with a letter specifying cause.

• ASPAD (formerlyCategory A SPAD) is where the train proceeds beyond its authorised movement to an unauthorised movement.^[12]
• ATechnical SPAR (formerlyCategory 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.
• ASignaller SPAR (formerlyCategory 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.
• ARunaway SPAR (formerlyCategory D SPAD) is where an unattended train or vehicles not attached to a traction unit run away past a signal at danger. Note that where this was the fault of the driver, this will be classed as a SPAD.

Some SPADs are defined as a;

• SAS SPAD – "Starting against signal" SPAD,^[13] where the train was standing at a danger signal and the driver moved past it.

• SOY SPAD – "Starting on yellow" SPAD,^[13] where the train started on a caution signal and the driver did not appreciate that the next signal would be at danger.

Signals form part of a complex system, and it is inevitable that faults may occur. They are designed tofail safe, so that when problems occur, the affected signal indicates danger (an example where this did not happen, known as awrong-side failure, was theClapham 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. There are two methods of obtaining that authority:^[14]

Once the train has been brought to a stand at a signal which is at danger, the driver should attempt to contact the signaller. If the signal cannot be cleared then the driver must obtain the signaller's authority to pass it at danger. Methods for contacting the signaller may includeGSM-R cab radio, signal post telephone ormobile phone.

The signaller can authorise a driver to pass a signal at danger when:^[15]

• The signal is defective or disconnected
• The signal cannot be cleared becausesignalling orlevel crossing equipment has failed
• The signal is to be passed at danger forshunting purposes
• The signal cannot be cleared because a train or movement which has reversed is then required to start from beyond that signal
• An electric train is to pass the signal protecting an isolated section and proceed towards the limiting point
• A train has been accepted using restricted acceptance because the line is clear only up to the home signal of the next signal box and the section signal cannot be cleared
• In an emergency, and then only when authorised by the signal box supervisor or Operations Control, so that a train carrying passengers can enter an occupied section to use a station platform
• Anengineering train is to move towards apossession, or leave a line under possession at an intermediate point
• A train is to pass the signal protecting engineering work to gain access to a station where the train is required to start back, or a line under single line working, or a siding
• The line is to be examined to check that it is clear
• A train is to proceed at caution through anabsolute block section from thesignal box in rear when a failed train has been removed
• A train is to enter the section after a train or vehicle that has proceeded without authority has been removed, or the front portion of a divided train has passed through the section
• A train is to enter the section to assist a failed train, evacuate passengers from a failed train, remove a portion of a divided train, or remove a train or vehicles that have proceeded without authority
• Single line working applies
• Working by pilotman or modified working applies

The driver and signaller must 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, they must obey its aspect, at which point they can revert to normal working.

If contact with the signaller cannot be made then the driver must not move the train, unless it is standing at one of the following signals:

• A signal controlled from a signal box that is closed onabsolute block line only.
• An automatic signal where local instructions permit it, e.g. signals within tunnels on theNorthern City Line.

After passing a signal at danger under their own authority, the driver must stop at the next signal (even if it is showing a proceed aspect) and inform the signaller of what they have done.

Whenever a signal is passed at danger the driver is required to "proceed with caution, stop short of any obstructions, and drive at speed that will enable you to stop within the distance which you can see to be clear". Failure to do this has caused the following collisions:

•   –Roseville, 1950
•   –Stratford (London Underground), 1953
•   –Coppenhall Junction, 1962
•   –Wrawby Junction, 1983
•   –Glenbrook, 1999
•   –Vittorio Emanuele (Rome Metro), 2006

Except wherepermissive working is in use,interlocking usually prevents a train from being signalled into a section that is already occupied. When operational needs require it, this can be overridden, and provided it is carried out in accordance with the rules this is a safe practice. However, failure to follow protocol can result in a collision:

•   –Nashville, TN, 1918
•   –Bucerdea, 1968
•   –Castlecary rail accidents, 1968
•   –Seer Green, 1981
•   –Jakarta, 1987
•    –Zoufftgen, 2006
•   –Szczekociny, 2012
•   –Bad Aibling, 2016
•   –Thessaly, 2023

• Ding-ding, and away, British slang for a guard incorrectly giving permission to a driver to start away from a platform against a red signal.

• Office of Rail Regulation
• UK Health and Safety Executive - Signals passed at danger - Archived 12 February 2006.