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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.
There are many reasons why rails break.
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.
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.
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.
Several different types of misaligned plain line tracks can cause or contribute to a derailment.
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.
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 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.
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.
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.
Several types of derailments can be caused by in-train forces.
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 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.
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.
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 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:
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.
Many railway accidents involve derailment, either as a direct cause, or as a consequence of an accident.