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In rail transport, a derailment is a type of train wreck that occurs when a rail vehicle such as a train comes off its rails. Although many derailments are minor, all result in temporary disruption of the proper operation of the railway system and they are a potentially serious hazard.
A derailment of a train can be caused by a collision with another object, an operational error (such as excessive speed through a curve), the mechanical failure of tracks (such as broken rails), or the mechanical failure of the wheels, among other causes. In emergency situations, deliberate derailment with or catch points is sometimes used to prevent a more serious accident.
During the 19th century derailments were commonplace, but progressively improved safety measures have resulted in a stable lower level of such incidents. A sampling of annual approximate numbers of derailments in the United States includes 3000 in 1980, 1000 in 1986, 500 in 2010, and 1000 in 2022.George D Bibel, Train Wreck – the Forensics of Rail Disasters, Hopkins University Press, Baltimore, 2012, Huimin Wu and Nicholas Wilson, Railway Vehicle Derailment and Prevention, in Handbook of Railway Vehicle Dynamics
Derailments in the United States
In the event of a broken or cracked rail, the rail running surface may be disrupted if a piece has fallen out, or become lodged in an incorrect location, or if a large gap between the remaining rail sections arises. 170 broken (not cracked) rails were reported on Network Rail in the UK in 2008, down from a peak of 988 in 1998/1999.
Derailment may take place due to excessive gauge widening (sometimes known as road spread), in which the sleepers or other fastenings fail to maintain the proper gauge. In lightly engineered track where rails are spiked (dogged) to timber sleepers, spike hold failure may result in rotation outwards of a rail, usually under the aggravating action of crabbing of bogies (trucks) on curves.
The mechanism of gauge widening is usually gradual and relatively slow, but if it is undetected, the final failure often takes place under the effect of some additional factor, such as excess speed, poorly maintained running gear on a vehicle, misalignment of rails, and extreme traction effects (such as high propelling forces). The crabbing effect referred to above is more marked in dry conditions, when the coefficient of friction at the wheel to rail interface is high.
Modern technologies have reduced the incidence of these failures considerably, both by design (specially the elimination of plain bearings) and intervention (non-destructive testing in service).
The vehicle wheelsets become momentarily unloaded vertically so that the guidance required from the flanges or wheel tread contact is inadequate.
A special case is heat related buckling: in hot weather the rail steel expands. This is managed by stressing continuously welded rails (they are tensioned mechanically to be stress neutral at a moderate temperature) and by providing proper expansion gaps at joints and ensuring that fishplates are properly lubricated. In addition, lateral restraint is provided by an adequate ballast shoulder. If any of these measures are inadequate, the track may buckle; a large lateral distortion takes place, which trains are unable to negotiate. (In nine years 2000/1 to 2008/9 there were 429 track buckle incidents in Great Britain).On Network Rail, so excluding certain "Metro" networks.Rail Accident Investigation Board (UK), Derailment of a Train at Cummersdale, Cumbria, 1 June 2009, Derby, England, 2010
The first concentration of levers for signals and points brought together for operation was at Bricklayer's Arms Junction in south-east London in the period 1843–1844. The signal control location (forerunner of the signalbox) was enhanced by the provision of interlocking (preventing a clear signal being set for a route that was not available) in 1856.Brian Solomon, Railroad Signaling, Voyageur Press, Minneapolis, MN, 2003,
To prevent the unintended movement of freight vehicles from sidings to running lines, and other analogous improper movements, trap points and derails are provided at the exit from the sidings. In some cases these are provided at the convergence of running lines. It occasionally happens that a driver incorrectly believes they have authority to proceed over the trap points, or that the signaller improperly gives such permission; this results in derailment. The resulting derailment does not always fully protect the other line: a trap point derailment at speed may well result in considerable damage and obstruction, and even a single vehicle may obstruct the clear line.
The most common obstructions encountered are road vehicles at level crossings (grade crossings); malicious persons sometimes place materials on the rails, and in some cases relatively small objects cause a derailment by guiding one wheel over the rail (rather than by gross collision).
Derailment has also been brought about in situations of war or other conflict, such as during hostility by Native Americans, and more especially during periods when military personnel and materiel was being moved by rail.Don DeNevi and Bob Hall, United States Military Railway Service America's Soldier Railroaders in WWII, 1992, Boston Mills Press, Erin, Ontario, .Christian Wolmar, Engines of War: How Wars Were Won & Lost on the Railways, Atlantic Books, 2010,
More sophisticated technologies in use nowadays generally employ couplings that have no loose slack, although there is elastic movement at the couplings; continuous braking is provided, so that every vehicle on the train has brakes controlled by the driver. Generally this uses compressed air as a control medium, and there is a measurable time lag as the signal (to apply or release brakes) propagates along the train.
If a train driver applies the train brakes suddenly and severely, the front part of the train is subject to braking forces first. (Where only the locomotive has braking, this effect is obviously more extreme). The rear part of the train may overrun the front part, and in cases where coupling condition is imperfect, the resultant sudden closing up (an effect referred to as a "run-in") may result in a vehicle in tare condition (an empty freight vehicle) being lifted momentarily, and leaving the track. This effect was relatively common in the nineteenth century.Colin Cole, Longitudinal Train Dynamics, in Handbook of Railway Vehicle Dynamics
On curved sections, the longitudinal (traction or braking) forces between vehicles have a component inward or outward respectively on the curve. In extreme situations these lateral forces may be enough to produce derailment.
A special case of train handling problems is overspeed on sharp curves. This generally arises when a driver fails to slow the train for a sharp curved section in a route that otherwise has higher speed conditions. In the extreme this results in the train entering a curve at a speed at which it cannot negotiate the curve, and gross derailment takes place. The specific mechanism of this may involve bodily tipping (rotation) but is likely to involve disruption of the track structure and derailment as the primary failure event, followed by overturning.
Fatal instances include the Santiago de Compostela derailment in 2013 and the Philadelphia train derailment two years later of trains traveling about . Both went at about twice the maximum allowable speed for the curved section of track.
A flange climbing derailment can result if the relationship between these forces, L/V, is excessive. The lateral force L results not only from centrifugal effects, but a large component is from the crabbing of a wheelset which has a non-zero angle of attack during running with flange contact. The L/V excess can result from wheel unloading, or from improper rail or wheel tread profiles. The physics of this is more fully described below, in the section wheel-rail interaction.
Wheel unloading can be caused by twist in the track. This can arise if the cant (crosslevel, or superelevation) of the track varies considerably over the wheelbase of a vehicle, and the vehicle suspension is very stiff in torsion. In the quasi-static situation it may arise in extreme cases of poor load distribution, or on extreme cant at low speed.
If a rail has been subject to extreme sidewear, or a wheel flange has been worn to an improper angle, it is possible for the L/V ratio to exceed the value that the flange angle can resist.
If weld repair of side-worn switches is undertaken, it is possible for poor workmanship to produce a ramp in the profile in the facing direction, that deflects an approaching wheel flange on to the rail head.
In extreme situations, the infrastructure may be grossly distorted or even absent; this may arise from a variety of causes, including earthwork movement (embankment slips and washouts), earthquakes and other major terrestrial disruptions, or deficient protection during work processes, among others.
The benefit of linked wheels derives from the conicity of the wheel treads—the wheel treads are not cylindrical, but cone. On idealised straight track, a wheelset would run centrally, midway between the rails.
The example shown here uses a right-curving section of track. The focus is on the left-side wheel, which is more involved with the forces critical to guiding the railcar through the curve.
Diagram 1 below shows the wheel and rail with the wheelset running straight and central on the track. The wheelset is running away from the observer. (Note that the rail is shown inclined inwards; this is done on modern track to match the rail head profile to the wheel tread profile.)
Diagram 2 shows the wheelset displaced to the left, due to curvature of the track or a geometrical irregularity. The left wheel (shown here) is now running on a slightly larger diameter; the right wheel opposite has moved to the left as well, towards the centre of the track, and is running on a slightly smaller diameter. As the two wheels rotate at the same rate, the forward speed of the left wheel is a little faster than the forward speed of the right wheel. This causes the wheelset to curve to the right, correcting the displacement. This takes place without flange contact; the wheelsets steer themselves on moderate curves without any flange contact.
The sharper the curve, the greater the lateral displacement necessary to achieve the curving. On a very sharp curve (typically less than about 500 m or 1,500 feet radius) the width of the wheel tread is not enough to achieve the necessary steering effect, and the wheel flange contacts the face of the high rail.The high rail is considered to be the outer rail in a curve; the low rail is the inner rail.
Diagram 3 shows the running of wheelsets in a bogie or a four-wheeled vehicle. The wheelset is not running parallel to the track: it is constrained by the bogie frame and suspension, and it is yawing to the outside of the curve; that is, its natural rolling direction would lead along a less sharply curved path than the actual curve of the track.Yaw describes the situation when the longitudinal axis of the wheelset is not the same as the longitudinal axis of motion.
The angle between the natural path and the actual path is called the angle of attack (or the yaw angle). As the wheelset rolls forward, it is forced to slide across the railhead by the flange contact. The whole wheelset is forced to do this, so the wheel on the low rail is also forced to slide across its rail.This was understood as early as 1844, when Robert Stephenson gave evidence that "in bringing round the curve, the wheels will all be fixed on the axles, and being of the same size, of course the outside has to go over more ground than the inside and therefore the outside ones slide upon the turn, and consequently, as you see in the Bristol stations where, you will see such wheels grind in their operation." Stephenson was giving evidence in the House of Commons regarding the South Devon Railway bill, on 26 April 1844, quoted in Hugh Howes, The Struggle for the South Devon Railway, Twelveheads Press, Chacewater, 2012,
This sliding requires a considerable force to make it happen, and the friction force resisting the sliding is designated "L", the lateral force. The wheelset applies a force L outwards to the rails, and the rails apply a force L inwards to the wheels. Note that this is quite independent of "centrifugal force".Centrifugal force is a convenient imaginary concept; strictly speaking it is the inertia of a body being accelerated, equal to the product of the mass of the body and the acceleration. However at higher speeds the centrifugal force is added to the friction force to make L.
The load (vertical force) on the outer wheel is designated V, so that in Diagram 4 the two forces L and V are shown.
The steel-to-steel contact has a coefficient of friction that may be as high as 0.5 in dry conditions, so that the lateral force may be up to 0.5 of the vertical wheel load.The value of L is determined by the load on both wheels of the wheelset multiplied by the coefficient of friction, plus the centrifugal force. But the sliding on the wheel on the low rail is not lateral—the wheel tread is actually sliding backwards (i.e rotating less rapidly than the forward speed requires) and the lateral friction force generated is limited by the vector of the sliding action.
During this flange contact, the wheel on the high rail is experiencing the lateral force L, towards the outside of the curve. As the wheel rotates, the flange tends to climb up the flange angle. It is held down by the vertical load on the wheel V, so that if L/V exceeds the trigonometrical tangent of the flange contact angle, climbing will take place. The wheel flange will climb to the rail head where there is no lateral resistance in rolling movement, and a flange climbing derailment usually takes place. In Diagram 5 the flange contact angle is quite steep, and flange climbing is unlikely. However, if the rail head is side-worn (side-cut) or the flange is worn, as shown in Diagram 6 the contact angle is much flatter and flange climbing is more likely.
Once the wheel flange has completely climbed onto the rail head, there is no lateral restraint, and the wheelset is likely to follow the yaw angle, resulting in the wheel dropping outside the rail. An L/V ratio greater than 0.6 is considered to be hazardous.
It is emphasised that this is a much simplified description of the physics; complicating factors are creep, actual wheel and rail profiles, dynamic effects, stiffness of longitudinal restraint at axleboxes, and the lateral component of longitudinal (traction and braking) forces.
With simple wagon derailments where the final position is close to the proper track location, it is usually possible to pull the derailed wheelsets back on to the track using rerailing ramps; these are metal blocks designed to fit over the rails and to provide a rising path back to the track. A locomotive is usually used to pull the wagon. A disadvantage of doing it this way is that the ramps can seriously damage the infrastructure. Because of which, this procedure may not be used in several countries.
If the derailed vehicle is further from the track, or its configuration (such as a high centre of gravity or a very short wheelbase) make the use of ramps impossible, jacks may be used. In its crudest form, the process involves lifting the vehicle frame and then allowing it to fall off the jack towards the track. This may need to be repeated.
A more sophisticated process involves a controlled process using slewing jacks in addition. This combination of lifting and sliding is called a hydraulic rerailing system. A system consisting of high pressure hydraulic lifting jacks (used for lifting the train) so a sliding system can be positioned underneath the vehicle. The sliding system consist of a beam (also called a bridge) with sleds or carriages which are moved laterally with a horizontally positioned high pressure hydraulic jack to push the vehicle back above track. After which it is lowered again on the track.
Photographs of early locomotives often indicate one or more jacks carried on the frame of the locomotive for the purpose, presumed to be a frequent occurrence.
When more complex rerailing work is needed, various combinations of cable and pulley systems may be used, or the use of one or more rail-borne cranes to lift a locomotive bodily.Peter Tatlow, Railway Breakdown Cranes: Volume 1, Noodle Books, 2012, Peter Tatlow, Railway Breakdown Cranes: Volume 2, Noodle Books, 2013, In special cases road cranes are used, as these have greater lifting and reach capacity, if road access to the site is feasible.
In extreme circumstances, a derailed vehicle in an awkward location may be scrapped and cut up on site, or simply abandoned as non-salvageable.
In the earlier Hither Green rail crash, a triangular segment of rail at a joint became displaced, and lodged in the joint; it derailed a passenger train and 49 persons died. Poor maintenance on an intensively operated section of route was the cause. Report on the Derailment that Occurred on 5th November 1967 at Hither Green in the Southern Region of British Railways, Ministry of Transport, Her Majesty's Stationery Office, London, 1968
There have been several other derailments in the UK due to trains entering speed-restricted sections of track at excessive speed; the causes have generally been inattention by the driver due to alcohol, fatigue or other causes. Prominent cases were the Nuneaton rail crash in 1975 (temporary speed restriction in force due to trackwork, warning sign illumination failed),Her Majesty's Railway Inspectorate, Report on the Derailment that occurred on 6th June 1985 at Nuneaton in the London Midland Region of British Railways, Her Majesty's Stationery Office, 1986 the Morpeth accident in 1984 (express passenger sleeping car train took restricted sharp curve at full speed; alcohol a factor; no fatalities due to the improved crashworthiness of the vehicles)Her Majesty's Railway Inspectorate, Report on the Derailment that occurred on 24th June 1984 at Morpeth in the Eastern Region of British Railways, Her Majesty's Stationery Office, 1985
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