History
The first recorded train derailment in history is known as the Hightstown Rail Accident in New Jersey that occurred on November 8, 1833. The train was traveling between Hightstown and Spotswood New Jersey and derailed after an axle broke on one of the carriages as a result of a journal box catching fire. The derailment resulted in one casualty and twenty-three injuries, and it was recorded that both New York railroad magnate Cornelius Vanderbilt and former U.S president John Quincy Adams were on the train as it took place, in which Adams wrote about the event in his journal. During the 19th century derailments were commonplace, but progressively improved safety measures have resulted in a stable lower level of such incidents. In the United States, derailments have dropped dramatically since 1980 from over 3,000 annually (1980) to 1,000 or so in 1986, to about 500 in 2010.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''Causes
Derailments result from one or more of a number of distinct causes; these may be classified as: * the primary mechanical failure of a track component (for example broken rails, gauge spread due to sleeper (tie) failure) * the primary mechanical failure of a component of the running gear of a vehicle (for example axlebox failure, wheel breakage) * a fault in the geometry of the track components or the running gear that results in a quasi-static failure in running (for example rail climbing due to excessive wear of wheels or rails, earthworks slip) * a dynamic effect of the track-vehicle interaction (for example extremeBroken rails
Broken rails are a leading cause of derailments. According to data from the Federal Railroad Administration, broken rails and welds are the most common reason for train derailments, making up more than 15 percent of derailment cases. A traditional track structure consists of two rails, fixed at a designated distance apart (known as theDefective wheels
The running gear — wheelsets, bogies (trucks), and suspension — may fail. The most common historical failure mode is collapse of plain bearings due to deficient lubrication, and failure of leaf springs; wheel tyres are also prone to failure due to metallurgical crack propagation. 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).Unusual track interaction
If a vertical, lateral, or crosslevel irregularity is cyclic and takes place at a wavelength corresponding to the natural frequency of certain vehicles traversing the route section, there is a risk ofImproper operation of control systems
Junctions and other changes of routing on railways are generally made by means of points (switches — movable sections capable of changing the onward route of vehicles). In the early days of railways these were moved independently by local staff. Accidents — usually collisions — took place when staff forgot which route the points were set for, or overlooked the approach of a train on a conflicting route. If the points were not correctly set for either route — set in mid-stroke — it is possible for a train passing to derail. 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.Derailment following collision
If a train collides with a massive object, it is clear that derailment of the proper running of vehicle wheels on the track may take place. Although very large obstructions are imagined, it has been known for a cow straying on to the line to derail a passenger train at speed such as occurred in theHarsh train handling
The handling of a train can also cause derailments. The vehicles of a train are connected by couplings; in the early days of railways these were short lengths of chain ("loose couplings") that connected adjacent vehicles with considerable slack. Even with later improvements there may be a considerable slack between the traction situation (power unit pulling the couplings tight), and power unit braking (locomotive applying brakes and compressing buffers throughout the train). This results in coupling surge. 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 theFlange climbing
The guidance system of practical railway vehicles relies on the steering effect of the conicity of the wheel treads on moderate curves (down to a radius of about 500 m, or about 1,500 feet). On sharper curves flange contact takes place, and the guiding effect of the flange relies on a vertical force (the vehicle weight). 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.Wheel-rail interaction
Nearly all practical railway systems use wheels fixed to a common axle: the wheels on both sides rotate in unison. Tramcars requiring low floor levels are the exception, but much benefit in vehicle guidance is lost by having unlinked wheels.Jean-Bernard Ayasse and Hugues Chollet, ''Wheel—Rail Contact'', in ''Handbook of Railway Dynamics'' The benefit of linked wheels derives from the conicity of the wheel treads—the wheel treads are not cylindrical, but conical. 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 here broad gauge trains were negotiating sharp curves 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 aRerailing
Following a derailment, it is naturally necessary to replace the vehicle on the track. If there is no significant track damage, that may be all that is needed. However, when trains in normal running derail at speed, a considerable length of track may be damaged or destroyed; far worse secondary damage may be caused if a bridge is encountered. 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 huge 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.Examples
''Note: there is a large list of railway accidents in general atPrimary mechanical failure of a track component
In thePrimary mechanical failure of a component of the running gear of a vehicle
In theDynamic effects of vehicle - track interaction
In 1967 in the UK there were four derailments due to buckling of continuously welded track ("CWR"): at Lichfield on 10 June, an empty carflat train (a train of flat cars for transporting automobiles); on 13 June an express passenger train was derailed at Somerton; on 15 July a freightliner train (container train) was derailed at Lamington; and on 23 July an express passenger train was derailed at Sandy. The official report was not entirely conclusive as to the causes, but it observed that the annual total of buckling distortions was 48 in 1969, having been in single figures in every previous year, and that eat-relateddistortions per 1,000 miles per annum were 10.42 for CWR and 2.98 for jointed track in 1969, having been a maximum of 1.78 and 1.21 in the previous ten years. 90% of the distortions could be attributed to one of the following: * failure to comply with the instructions for laying or maintaining CWR track * recent interference with the consolidation of the ballast * the effect of discontinuities in the CWR track such as points etc. * extraneous factors such as formation subsidence.Major C F Rose, ''Railway Accidents, Interim Report on the Derailments that occurred on Continuous Welded Track at Lichfield (London Midland Region), Somerton (Western Region) and Sandy (Eastern Region), British Railways, during June and July 1969, and on the General Safety of this form of Track'', Her Majesty's Stationery Office, London, 1970Improper operation of control systems
In the Connington South rail crash on 5 March 1967 in England, a signaller moved the points immediately in front of an approaching train. Mechanical signalling was in force at the location, and it was believed that he improperly replaced the signal protecting the points to danger just as the locomotive passed it. This released the locking on the points and he moved them to lead to a loop line with a low speed restriction. The train, travelling at , was unable to negotiate the points in that position and five people died.Lt-Col I K A McNaughton, ''Report on the Derailment that Occurred on 5th March, 1967, at Connington South in the Eastern Region British Railways'', Her Majesty's Stationery Office, London, 1969Secondary events following collision
A passenger train was derailed in theTrain handling effects
The Salisbury rail crash took place on 1 July 1906; a first class only special boat train from Stonehousepool, Plymouth England, ran through Salisbury station at about ; there was a sharp curve of ten chains (660 feet, 200 m) radius and a speed restriction to . The locomotive overturned bodily and struck the vehicles of a milk train on the adjacent line. 28 people were killed. The driver was sober and normally reliable, but had not driven a non-stopping train through Salisbury before.Major J W Pringle, Report for the Board of Trade, London, 31 July 1906See also
*Notes
References
Further reading
* {{Rail accidents Rail technologies Railway accidents and incidents