A signal passed at danger (SPAD), known in the United States as a stop signal overrun (SSO)[1] and in Canada as passing a stop signal,[2] is an event on a railway where a train passes a stop signal without authority.[3] This is also known as running a red.[4]
The name derives from red colour light signals and horizontal semaphore signals in the United Kingdom, which are said to be at danger when they indicate that trains must stop (also known as the signal being on). This terminology is not used in North America where not all red signals indicate stop. In the UK, a signal 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 high inertia of trains, and the low adhesion between the wheels and track, means it takes a long distance for the train 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 the Ladbroke Grove rail crash.
The causes and prevention of SPADs is actively researched. Causes of SPADs are always multidimensional. Some of the causes of SPADs are:
See main article: article and Automatic train protection.
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 and Chiltern Railways) are fitted with this equipment.
See main article: article and Driver's reminder appliance.
The 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,[9] 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 designated overlap beyond that signal, a collision should not occur.
See main article: article and Train stop.
On the London Underground (for example), mechanical train stops are fitted beside the track at signals to stop a train, should an S.P.A.D occur.
Train stops are also installed on main line railways in places where tripcock equipped trains run in extensive tunnels, such as the on the Northern City Line where the Automatic warning system and Train Protection & Warning System are not fitted.
See main article: article and Automatic Warning System.
On the UK mainline, AWS consists of an on-board receiver/timer connected to the emergency 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 restrictive signal aspect.
See main article: article.
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 activate the train's emergency brake if it approaches faster than the 'trigger speed' when the signal is at danger. 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 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.
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[10] 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 of TPWS 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 a proceed 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. A SPAD is where the train proceeds beyond its authorised movement to an unauthorised movement. Other types are categorised as SPAR ("signal passed at red").
Prior to December 2012,[11] the term "SPAD" applied to all such incidents, with a letter specifying cause.
Some SPADs are defined as a;
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, known as a wrong-side failure, 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. There are two methods of obtaining that authority:[13]
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 include GSM-R cab radio, signal post telephone or mobile phone.
The signaller can authorise a driver to pass a signal at danger when:[14]
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:
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.
| Country | Incident | Year | |
|---|---|---|---|
| United States | 1853 | ||
| United Kingdom | 1857 | ||
| Canada | 1864 | ||
| United Kingdom | 1887 | ||
| Denmark | 1897 | ||
| United Kingdom | 1898 | ||
| United Kingdom | 1900 | ||
| United States | 1906 | ||
| Australia | 1908 | ||
| United Kingdom | 1909 | ||
| United Kingdom | 1913 | ||
| United Kingdom | 1915 | ||
| Hungary | 1916 | ||
| United Kingdom | 1928 | ||
| Germany | 1939 | ||
| United Kingdom | 1940 | ||
| United Kingdom | 1941 | ||
| United States | Lackawanna Limited wreck | 1943 | |
| United Kingdom | 1946 | ||
| United Kingdom | 1952 | ||
| United Kingdom | 1955 | ||
| United Kingdom | 1957 | ||
| United Kingdom | 1958 | ||
| United States | 1958 | ||
| United Kingdom | 1962 | ||
| Netherlands | 1962 | ||
| United Kingdom | 1969 | ||
| Australia | 1969 | ||
| United Kingdom | 1979 | ||
| United Kingdom | 1979 | ||
| United States | 1979 | ||
| Poland | 1980 | ||
| United Kingdom | 1984 | ||
| United Kingdom | 1984 | ||
| Canada | 1986 | ||
| United Kingdom | 1986 | ||
| United States | 1987 | ||
| United Kingdom | 1989 | ||
| United Kingdom | 1989 | ||
| Germany | 1990 | ||
| Japan | 1991 | ||
| United Kingdom | 1991 | ||
| United Kingdom | 1994 | ||
| Canada | 1995 | ||
| Germany | 1995 | ||
| United States | 1996 | ||
| United States | 1996 | ||
| Australia | 1996 | ||
| United Kingdom | 1997 | ||
| Australia | 1997 | ||
| Finland | 1998 | ||
| United Kingdom | 1999 | ||
| United Kingdom | 1999 | ||
| United Kingdom | 1999 | ||
| Norway | 2000 | ||
| Belgium | 2001 | ||
| United Kingdom | 2003 | ||
| Egypt | 2006 | ||
| Netherlands | 2006 | ||
| United States | 2008[15] | ||
| Belgium | 2010 | ||
| India | 2010 | ||
| Indonesia | 2010 | ||
| Germany | 2011 | ||
| Netherlands | 2012 | ||
| United States | 2012 | ||
| Switzerland | 2013 | ||
| Romania | 2014[16] | ||
| Belgium | 2016 | ||
| Portugal | 2020[17] | ||
| United Kingdom | 2021[18] |
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), 2006Except where permissive 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:
– Bucerdea, 1968 – Castlecary rail accidents, 1968 – Seer Green, 1981
– Jakarta, 1987
– Zoufftgen, 2006
– Szczekociny, 2012
– Bad Aibling, 2016
– Thessaly, 2023