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A derailment is an incident on a railway or tramway in which one or more rail vehicles leave the tracks on which it is, or they are, travelling. A derailment is defined as an event in which the proper guidance of railway vehicles is disrupted.
There are several principal causes of derailment: broken or misaligned rails, excessive speed (especially on curves), faults in the train and its wheels, and faults in sets of points. Derailment can also occur as a secondary effect in the aftermath of a collision.
Trap points and derails are designed to protect main lines from runaway trains by deliberately derailing them, preventing them from threatening traffic on main lines; improper movement through these safeguards may derail a train or vehicles.
Rail breakages 
There are many reasons why rails break.
Rail breaks at rail joints 
In jointed track, rails are usually connected with bolted fishplates. The web of the rail experiences large shear forces and this is enhanced around the bolt hole. 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.
Manufacturing defects in rail 
Recent rail-making processes have also been trending toward the production of harder rail steel, for durability. This has had the effect of reducing the rate of surface wear, to the extent that micro-cracking at the surface due to fatigue propagates faster than the wear of the rail surface; this has resulted in catastrophic fatigue cracking.
Other 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.
When a locomotive wheel spins without moving the train forward (also known as slipping), the small section of rail directly under the wheel is heated by the forces of friction between the wheel and itself. The wheel rests on an area of rail about two centimeters long, so the heating effect is very localized and occurs very quickly. The heated spot is quenched very quickly, resulting in undesirable changes to the steel metallurgy, often resulting in embrittlement as well as a surface discontinuity which causes localised impact forces.
If the brakes are dragging or the axle ceases to move on a rail vehicle while the train is in motion, the wheel will be dragged along the head of the rail, causing a 'flat spot' to develop on the wheel surface where it contacts the rail. When the brakes are subsequently released, the wheel will continue to roll around with the flat spot, causing a banging noise with each rotation. This condition is known as wheel out of round (US) or a wheel flat (UK). The impact of flat wheels on the rail causes a hammering action that produces higher dynamic forces than a simple passage of a round wheel; these forces can exacerbate a weak rail condition and cause a rail break.
In continuously welded rail (CWR), the track structure is designed to be stable under compression during the summer heat, and under tension during the winter. The welded rail cannot expand or contract lengthwise, resulting in substantial tension or compression of the rail section. Cold-weather tension, if sufficiently large, can cause a poorly fastened joint or weld to pull apart.
Methods to detect rail breaks 
If a rail breaks cleanly, it may be detected by electrical track circuit indications, depending on the type of train location system employed by the control system in use. However there are many situations in which this is not applicable technically, and of course if the rail is badly damaged but not broken, the indication will not occur.
Typically, this partial type of rail break is detected by the visual inspection of a track worker 'walking the line', or by ultrasonic testing, in which the reflection of an ultrasonic pulse is measured (analogous to radar). Ultrasonic testing is accomplished by running a detector car over the tracks, or by hand held equipment.
Misaligned railroad tracks 
Several different types of misaligned plain line tracks can cause or contribute to a derailment.
Wide gauge derailments 
These occur when the rails are allowed to be significantly wider apart than the proper track gauge under load, resulting in one or more wheelsets dropping into the space between the rails. While the intuitive cause of this phenomenon is the pushing outward of the high rail on curves due to dynamic effects, the real cause is the crabbing effect of the wheelset steering on sharp curves (when the wheel tread coning is insufficient to effect the necessary steering). The high rail is forced outward by the force equal and opposite to the force necessary to push the low rail wheel laterally across its rail head. Estimation of the friction force available is complicated by the simultaneous backward creep of the low rail wheel -- the lateral component of the vector force is reduced. Counter-intuitively, this effect is almost independent of speed.
This track failure is usually caused by hot weather and inadequate geometric retention of track geometry; during hot weather the rails expand and if not properly restrained, can force the track significantly out of alignment. This phenomenon may take place in jointed track (if fishplates are not properly lubricated) or in continuously welded track that is not properly pre-stressed. It is especially likely on very sharp curves, and in situations where the ballast does not afford adequate support.
Incorrect track geometry 
Derailment due to incorrect track geometry takes place when the alignment of the track is catastrophically defective; this is coupled with crosslevel errors on curves where the relationship between crosslevel and curvature is seriously inappropriate.
In a washout situation, the entire support system for the track is rendered useless, generally as a result of earthworks being removed or impaired due to floodwater action.
Dynamic effects 
Dynamic failures tend to be more insidious: 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. 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 – in reality unloading wheelsets so that the guidance required from the flanges or wheel tread contact is inadequate.
Rail profile problems 
These irregularities can lead to derailment if seriously incorrect; the most common situations are when there is serious sidewear and also misaligned joints in sharply curved, slow speed situations; and when the switch rail profile has been incorrectly altered during repair welding, creating a ramp for trains in the facing direction when the switch rail is closed.
Rail rotation 
Unconstrained rotation can take place when inadequate holding-down forces are available, allowing the rail to twist outward, resulting in a gross increase in gauge. This is most likely in very low-grade track when very poor sleeper quality is present.
In-train forces 
Several types of derailments can be caused by in-train forces.
Uneven loading 
This type of derailment can occur in freight trains if empties (unloaded railcars) are marshalled in train between the locomotive and heavy loaded cars. For example, if the consist contains locomotives, empty trailer racks, followed by a large block of loaded coal hoppers. When the train is braking, brakes on the head end of the train will apply first causing the locomotive to slow down and the slack to run in. The heavy coal cars towards the end of the train would shove the lighter cars forward with considerable force. This can cause the lighter cars to arch upwards and jump the tracks, especially if the in train forces cause couplers to overload.
This type of derailment occurs when a string of light cars travel over a reverse curve (S-curve) while locomotives are attempting to accelerate with all slacks pulled out. The reverse curve offers considerable resistance to the locomotive. The cars would tend to prefer to travel in a straight line, the line of least resistance. This causes in-train forces towards the inside of the curve in the middle of the train. If the middle cars are too light, wheels may climb the inside of the curve and travel along a geometric chord to the arc.[dubious ]
Poor train handling 
Poor train handling techniques can cause derailment, regardless of the load. Usually, allowing the slack to run in too fast (while braking or at the bottom of a valley) is the cause of derailment in cases relating to poor train handling. Over hill terrain, experienced train engineers will run the train with dynamic brakes while keeping the slack under control. Air brakes are usually only used to bring the train to a complete stop at low speeds.
A static balance issue arises in unevenly loaded timber cars. Timber centerbeam flatcars are to be loaded with equal amount of timber on both sides. However, unloading only takes place on one side of the car at a time, which requires the half-loaded car to be run around a wye track to allow the shipper to gain access to the other side of the car. While the car is being run around, the center of gravity of the car is on one side. If crossovers or curves are traversed at too high a speed, the car can easily topple over onto its heavy side.
Slow-speed derailments 
There are some derailments because of slow speed in tight curves, especially in freight trains with high center of gravity. The main reason for this phenomenon is unloading in the outer wheel, which goes to a critical situation because of the larger superelevation that creates an inward acceleration, resulting in an unloading.[further explanation needed] Because of the action of outer wheel as the steering force, this can lead to the climbing of wheel according to the Nadal formula, which expresses the relation between the lateral forces on the wheel and the vertical downforce of the wheel on the rail.
Rolling stock design 
Some strange failure modes have been recorded in the history of railroading. The L Class tank locomotives of the London, Brighton and South Coast Railway were found to be prone to derailment at high speeds due to water surging in the long sidetanks[dubious ]. The class was redesigned to incorporate an additional water tank between the frames and the capacity of the sidetanks was restricted to lower the centre of gravity Similarly, Amtrak's first long distance diesel locomotive, the EMD SDP40F, was implicated in certain crossover-related derailments. Investigations revealed that the location of a water tank within the locomotive may have caused excessive swaying while the locomotive traversed crossovers at high speeds, shifting the locomotive's center of gravity and forcing it to overturn onto its side.
Wheel and truck failures 
Wheel fracture derailments are quite rare. In the US, this may be partly due to the Federal Railroad Administration's requirement for 1,000-mile (1,600 km) undercarriage inspections for trains.[dubious ] Also, a variety of defect detectors en route may warn of most wheel and truck failure precursor conditions. Some reasons for wheel and truck failures are:
- Hot axlebox. This has been almost eliminated as freight car (goods wagon) trucks are transitioned from a simple bearing to a roller bearing design.
- Fracture of axle. Some freight train derailments have been caused by axle fractures, but these are relatively rare events.
- Fracture of wheel. This is also a rare event. However, the failure mode received a great deal of attention due to the InterCity Express (ICE) train's wreck in Eschede, Germany. The composite wheel then used on the ICE, which includes a rubber inner tire, failed catastrophically, resulting in a 100 mph (160 km/h)+ derailment that sent a train into a support pillar for a highway overpass. The overpass crashed down on top of the train, causing many fatalities.
At present, several technologies are available to detect abnormal wheel and truck conditions:
Trains can, but do not always, derail if they hit obstacles on the tracks, like animals, fallen branches, vehicles and bikes on level crossings, and so on. Once one locomotive or wagon derails, it becomes an obstacle for following wagons, leading to a pileup. The shape of the front of the train is important. If it is curved like a "cowcatcher", then obstacles may sometimes be thrown safely off to one side.
Trains can be derailed or tipped over by earthquakes. In Japan, JR East actively conducts research to prevent earthquake related derailments, especially of Shinkansen ("bullet") trains, by developing emergency communications systems that send a "train stop" signals to all trains when a heavy earthquake is detected. This permits the train to come to a safe stop if it is not already derailed by the primary shock, rather than allowing trains to continue running and potentially hitting a deformed structure or track segment.
Derailments have also been caused by deliberate means, usually during wartime or by bandits.
In the American frontier era, the transcontinental railroads were a target of Native American attacks, including derailment attempts. During World War II, the French Resistance made various sabotage attempts against Nazi supplies being transported by rail. The 2002 Jaunpur train crash and Rafiganj train disaster in India were suspected to be the work of militants.
Since engines and wagons are quite heavy, up to 300 short tons (268 long tons; 272 t), even a slight derailment can be difficult to rectify. In the US, minor low speed derailments are sometimes rerailed by the engine crew. Wooden blocks, planks, and metal bars can be used for this purpose. More serious derailments where the cars are completely removed from the normal track alignment will likely incur track damage, and vehicles may have to be removed by rail mounted or other cranes. If rolling stock rolls down an embankment as a result of a derailment, a locomotive and cable can sometimes be used to haul those vehicles back to the top again. In some cases, cars are simply left in the field after the derailment, because the cost of retrieval exceeds the economic value of the car. However, this can be done only if the abutter does not object.
Contracting companies specializing in derailment recovery exists in both UK and the US, and smaller railroads often rely on external contractors for disaster recovery.
Example accidents 
Many railway accidents involve derailment, either as a direct cause, or as a consequence of an accident.
19th Century 
- November 8, 1833 – Hightstown, New Jersey, United States: Hightstown rail accident - Carriages of a Camden & Amboy train derail at 25 miles per hour (40 km/h) in the New Jersey meadows between Spotswood and Hightstown when an axle breaks on a car due to an overheated journal. One car overturns, killing two and injuring 15. Among the survivors is Cornelius Vanderbilt, who will later head the New York Central Railroad. He suffers two cracked ribs and a punctured lung, and spends a month recovering from the injuries. Uninjured in the coach ahead is former U.S. President John Quincy Adams, who continues on to Washington, D.C. the next day.
- January 6, 1853 – Andover, Massachusetts, United States: The Boston & Maine noon express, traveling from Boston to Lawrence, Massachusetts, derails at 40 miles per hour (64 km/h) when an axle breaks at Andover, and the only coach goes down an embankment and breaks in two. Only one person is killed, the 12-year-old son of President-elect Franklin Pierce, but it is initially reported that General Pierce is also a fatality. He is on board, but is only badly bruised. The baggage car and the locomotive remain on the track.
- April 16, 1853 – Cheat River, Virginia (now West Virginia), United States: Two Baltimore & Ohio passenger cars tumble down a 100-foot ravine above the Cheat River in Virginia (now West Virginia), west of Cumberland, Maryland, after they are derailed by a loose rail.
20th Century 
- December 12, 1917 – Saint-Michel-de-Maurienne, France: A troop train derails near the entrance to the station after running away down a steep gradient from the entrance of the Fréjus Rail Tunnel; brake power was insufficient for the weight of the train. Around 800 deaths were estimated, with 540 officially confirmed. This was the world's worst-ever derailment, and worst rail disaster up to the end of the 20th century.
- July 2, 1922 – Winslow, Camden County, New Jersey, United States: The Owl, a Reading Railroad train derailment, at Winslow Junction on the West Jersey and Seashore Line tracks near the Winslow Tower. Shortly before midnight, train 33 derails when the seashore-bound locomotive going more than 90 miles per hour (140 km/h) speeds through an open switch. Four passengers, the engineer, fireman and conductor were killed.
- Jamaica July 30, 1938 – near Balaclava Station, Jamaica: five overcrowded cars derail; 32 killed, 70 injured.
- February 18, 1947 – Blair County, Pennsylvania, United States: The Red Arrow, a Pennsylvania Railroad express passenger train, jumps off the track on the Bennington Curve near Altoona, Pennsylvania and tumbles down a large hill, resulting in 24 deaths and 131 injuries.
- Stein-Saeckingen 1991 - 8 tank cars derailed
- Lausanne 1994 - 15 tank cars derailed
- Eschede train disaster June 3, 1998 - The world's deadliest high-speed train accident - 101 dead.
- October 16, 1999 - Near Ludlow, California: Amtrak’s westbound Southwest Chief passenger train, en route from Chicago to Los Angeles, was derailed while crossing the Mojave Desert 126 miles (203 km) northeast of Los Angeles when the train reached a section of track that had been damaged by the 7.1-magnitude Hector Mine earthquake, which had occurred 24 minutes prior to the derailment. Four of the 155 passengers on the train suffered minor injuries in the incident.
21st Century 
- 2000 – Hatfield rail crash.
- 10 May 2002 – Potters Bar rail crash, Potters Bar, England, United Kingdom: A points failure causes a British Rail Class 365 to derail on the approach to Potters Bar railway station. As a result, the train slides sideways across the station platform, killing six on the train and one under the road bridge.
- January 31, 2003 – Waterfall train disaster, Waterfall, New South Wales, Australia: A train derails as it rounds a sharp curve rated for 60 km/h at a speed of 117 km/h, after the train driver has a heart attack. The two safety mechanisms—the driver's deadman's brake, which remains depressed because of the driver's weight, and the guard who could have applied the emergency brake, but is in a microsleep at the time—are found to be the direct causes of the incident.
- 23 February 2007 – Grayrigg derailment, Grayrigg, England, United Kingdom: The 17:15 Virgin West Coast Pendolino service from London Euston to Glasgow Central, travelling on the West Coast Main Line, derails due to stretcher bar disconnection.
- April 28, 2008 – Jiao-Ji line derailment, Shandong, China: The T195 Express service from Beijing to Qingdao derails at Shandong due to excessive speed, and collides moments later with another passenger train traveling in the opposite direction, killing over 70 passengers and railroad maintenance workers, and injuring more than 400.
- February 13, 2009 - Orissa train derailment a passenger train derailment that occurred at 19:45 local time (14:15 UTC) in the dark in the eastern state of Orissa, India, on 13 February 2009. Nine people were killed and 150 people were injured in the incident.
- 23 February 2009 - Limpopo
- June 29, 2009 - Viareggio train derailment - Derailment of a freight train and subsequent fire . Twenty-six people were injured and 32 people were confirmed as having died.
- June 30, 2010 A coal train derailment in Wayzata, MN. No injuries.
- October 25, 2010 – NJ Transit train 6621 derailed departing Penn Station New York. 300 passengers were offloaded and no injuries were reported.
- November 24, 2010 - Machynlleth, UK. A British Rail Class 153 derailed whilst operating a service between Birmingham and Aberystwyth, however there were no reported injuries.
- July 16, 2011 - Fridley, MN, USA. Several cars and locomotives of a BNSF freight train derails following a bridge collapse due to washout from heavy rain earlier that morning, spilling its cargo of corn. There had been two minor injuries reported. 
- February 26, 2012 - Burlington derailment, Canada. A Via Rail train derailed carrying 75 passengers. 46 people were injured and 3 people were killed.
- July 4, 2012 - Glenview, Il, USA. Several cars of a Union Pacific freight train derails followed by a partial bridge collapse. There have been at least two casualties reported.  
- August 21, 2012 - Ellicott City, MD, USA. 21 cars derailed or overturned, damaging property and cars near Main Street Ellicott City. Two people apparently on the train tracks were killed during the derailment. 
- October 26, 2012 - Bicol Express train, bound for Ligao City was derailed a little past midnight Friday after heavy rains softened the track’s embankment in a flooded area of Sariaya, Quezon, injuring eight of the 128 passengers rescued from the wreckage. The PNR said three coaches of the five-car train set T611 headed for Ligao City derailed around 12:14 a.m. 
See also 
- Lists of rail accidents
- Classification of railway accidents
- Train wreck
- Tram accident
- Kenya Railways Corporation - accidents
- American Experience: Native Americans and the Transcontinental Railroad
- "Via Rail derailment: Train leaves a path of destruction".