In rail transport, a derailment is a type of railway accident that occurs when a moving 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, operator error (such as excessive speed through a curve), mechanical failure of tracks (such as broken rails) or wheels, among other causes. In emergency situations, deliberate derailment with derails or catch points is sometimes used to prevent a more serious accident.
History
United States The first recorded train derailment in history is known as the Hightstown rail accident in New Jersey that occurred on 8 November 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 fatality 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. 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 Although considerably less common than they were a century or even half a century ago, major incidents do still occur from time to time. Besides operator error, system failure due to aging infrastructure, often combined with constant or even increasing demand, has played a growing role in accidents, especially along the busiest corridors. In the past two decades, notable recent derailments include:
East Palestine, Ohio, train derailment - February 3, 2023: Norfolk Southern freight train transporting chemicals derails in East Palestine, Ohio. The spillage of hundreds of thousands of pounds of hazardous materials causes an environmental disaster. The National Transportation Safety Board (NTSB) concluded that the accident was the result of an overheated wheel bearing. On March 4th another Norfolk Southern train derailed in Ohio, although railways officials stressed that this time it was not carrying any toxic chemicals and that its contents posed no threat to either the environment or the general public. Spuyten Duyvil derailment - December 1, 2013: Metro-North commuter train with 116 passengers derails from a curve in the Spuyten Duyvil neighborhood of the Bronx, a borough of New York City, causing 4 deaths and at least 61 injuries. According to the NTSB, the train had been traveling at 82 mph (132 kph) at the time of the accident, far exceeding the 30 mph (48 kph) speed restriction for the curve. The NTSB concluded that this had been the result of operator error, with the engineer having fallen asleep due to a sleep disorder and a recent schedule shift.
Incidents over time A line chart plotting derailments per year, for both freight and passenger railroad operations in the United States since 1975.
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 extreme Hunting oscillation, vertical bounce, track shift under a train, excessive speed) improper operation of points, or improper observance of signals protecting them (signal errors) as a secondary event following collision with other trains, road vehicles, or other obstructions (level crossing collisions, obstructions on the line) train handling (snatches due to sudden traction or braking forces, referred to as slack action in North America).
The U.S. Federal Railroad Administration categorises derailments differently, for the use of professionals in the industry; these are not completely helpful for external readers, but for completeness, the main groupings are given here:
Rail, joint bar and anchoring Track geometry defect General switching rules Wheels Axles and journal bearings Switches Frogs, switches and track appliances Bogie (truck) components Train handling / train make-up Highway rail grading
Source: Safety Database Analysis, Transportation Technology Center Inc, Pueblo Col, 2002, quoted in Wu and Wilson, page 210-211
Broken 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 the track gauge), and supported on transverse sleepers (ties). Some advanced track structures support the rails on a concrete or asphalt slab. The running surface of the rails is required to be practically continuous and of the proper geometrical layout.
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.
In jointed track, rails are usually connected with bolted fishplates (joint bars). The web of the rail experiences large shear forces and these are enhanced around the bolt hole. Where track maintenance is poor, metallurgical fatigue can result in the propagation of star cracking from the bolthole. In extreme situations this can lead to a triangular piece of rail at the joint becoming detached. Metallurgical changes take place due to the phenomenon of gauge corner cracking (in which fatigue microcracking propagates faster than ordinary wear), and also due to the effects of hydrogen inclusion during the manufacturing process, leading to crack propagation under fatigue loading. Local embrittlement of the parent metal may take place due to wheel spin (traction units rotating driving wheels without movement along the track). Rail welds (where rail sections are joined by welding) may fail due to poor workmanship; this may be triggered by extremely cold weather or improper stressing of continuously welded rails, such that high tensile forces are generated in the rails. The fishplates (joint bars) in jointed track may fail, allowing the rails to pull apart in extremely cold weather; this is usually associated with uncorrected rail creep.
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.
Defective 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 of resonant harmonic oscillation in the vehicles, leading to extreme improper movement and possibly derailment. This is most hazardous when a cyclic roll is set up by crosslevel variations, but vertical cyclical errors also can result in vehicles lifting off the track; this is especially the case when the vehicles are in the tare (empty) condition, and if the suspension is not designed to have appropriate characteristics. The last condition applies if the suspension springing has a stiffness optimised for the loaded condition, or for a compromise loading condition, so that it is too stiff in the tare situation.
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
Improper 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,