A derailment is an accident on a railway or tramway in which a rail vehicle, or part or all of a train, leaves the tracks on which it is travelling, with consequent damage and in many cases injury and/or death.
There are several main causes of derailment: broken or misaligned rails, excessive speed, faults in the train and its wheels, and collisions with obstructions on the track. Derailment can also occur as a secondary effect in the aftermath of a collision between two or more trains. Trap points protect main lines from runaway vehicles by deliberately derailing them to bring them to a stop.
There are many reasons why rail tracks break. In bygone days, it was common for a rail break to start near the joint between discrete rail segments. Manufacturing defects in rail can cause fissures. Wheelburns can also contribute to rail breaks by changing the metallurgy of a rail. Rails are also more likely to break when the weather is cold, when the ballast and ties/sleepers are not providing as much support as they should, and when ground or drainage condition is such that 'pumping' occurs under heavy load. All of these conditions can contribute to a broken rail, and in turn a possible derailment. Recently, the 'gauge corner cracking' phenomenon has come under the spotlight after a GNER high-speed train derailed in 2000 near Hatfield, England.
Rail breaks at rail joints
Each rail segment is 39 feet (11.9 m) long, and fishplates must be used to join them together. Rail joined with fish plates is known as jointed-rail or jointed track. The method to join two pieces of rail together is to drill two or three holes on the web of the rail at each segment-end, and bolt the two rail segments together using two fishplates, one on either side. The bolts and the area of rail around the drilled holes endure huge stresses as train wheels pass over the joint. If the rail joint is not properly supported by railroad tie and ballast underneath, the stresses may be even greater. Over time, the cumulative action of many wheel passages can cause a crack to appear. It is quite common for the crack to begin at the bolt holes. Cracks can also begin internally within the rail. Once began, the crack can travel within the rail, eventually finding its way to a surface, causing a piece of rail to break off.
Manufacturing defects in rail
The quality of rail steel has improved dramatically since the early days of railroading. The trend toward using continuously welded rail (CWR) requires a higher quality rail, due to the cyclic thermal expansion and contraction stresses that a CWR would be required to endure. In addition, rail operations in general have been trending toward higher speed and higher axle-load operation. Under these operating conditions, rail pieces rolled in the 19th century would likely break at an unacceptable rate. Despite the improved rail quality and rail metallugry, if impurities find their way into rail steel and are not detected by the quality assurance process, they can cause rail breaks under certain conditions.
Recent rail-making processes have also been trending toward a harder rail, requiring less frequent replacements under heavy loads. This has the side-effect of making the rail more brittle, and thus more susceptible to brittle fracture rather than plastic deformation. It is therefore imperative that unintentional impurities in rail be minimized. Tata of Holland and England, and U.S. Steel of Pittsburgh, are two current rail manufacturers.
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. While wheelburn typically does not cause the entire rail section to melt, it does heat the steel to red-hot temperatures. As the locomotive stops slipping and starts moving—or worse still, slips forward by a matter of inches and heats a different piece of rail—the heated spot cools down very quickly to normal temperature, especially when the weather is cold.
This heat-quench process results in annealing of the rail steel and causes substantial changes to its physical property. It can also cause internal stresses to form within the steel structure. As the rail surface cools, it may also become oxidized, or undergo other chemical changes by reacting with impurities that are on the surface of the rail. The net result of this process is that an area of the rail that is more susceptible to breakage is created.
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.
The banging of flat wheels on the rail causes a hammering action that produces higher dynamic forces than a simple passage of a round wheel. These dynamic forces can exacerbate a weak rail condition and cause a rail break.
In continuously welded rail (CWR), the ribbons of rail are designed to survive under compression during the summer heat, and under tension during the winter. The welded rail cannot expand or contract lengthwise, thus must deal with temperature-related physical expansion and contraction by changing cross-sectional area. During cold weather, this results in substantial tension along the direction of travel.
This tension, if sufficiently large, will cause a crack to develop at the weakest point in the rail. As previously discussed, the weak point could be caused by a manufacturing defect, a wheelburn, a poor weld, or some other irregularity in the rail. During exceptionally cold weather, the rail may break cleanly across, and a large gap may open up between two sections of formerly welded rail. This condition can easily cause a derailment under load.
The tension in the rail is amplified if a train rolls over the rail and brakes. A decelerating train has a tendency to pull the rails forward, resulting in increased tension in the part of the rail that follows directly beneath the rail-wheel interface. Part of this problem is mitigated by the use of rail anchors, which grips the rail at the bottom and anchors it to a railroad tie. The rail anchors prevent the rail from slipping longitudinally (along the direction of travel) and also serve to ensure the thermal stresses are evenly distributed along the CWR sections.
Methods to detect rail breaks
If a rail breaks cleanly, it is relatively easy to detect. A track occupancy light will light up in the signal tower indicating that a track circuit has been interrupted. If there is no train in the section, the signaler must investigate. One possible reason is a clean rail break. For detecting the rail break this way, one has to use signal bonds that are welded or pinbrazed on the head of the rail. If one uses signal bonds that are on the web of the rail, one will have a continued track circuit.
If a rail is merely cracked or has an internal fissure, the track circuit will not detect it, because a partially-broken rail will continue to conduct electricity. Partial breaks are particularly dangerous because they create the worst kind of weak point in the rail. The rail may then easily break under load—while a train is passing over it—at the point of prior fissure.
Typically, these type of rail breaks are detected by the visual inspection of a track engineer walking the line, or ultrasonic testing. Ultrasonic testing is accomplished by running a detector car over the tracks. Invented by Elmer Ambrose Sperry in the early 1900s, the detector car initially used induction to detect cracks within the steel. Later, ultrasonics were introduced and have remained the industry standard for detecting defects within rail. It works by sending an ultrasonic signal into the rail, which detects characteristic patterns in the reflected ultrasound since anomalies within the steel reflect ultrasonic energy. In effect, the testing device works like a Sonar that could 'see' internal crack and defects within the rail.
Misaligned railroad tracks
Several different types of misaligned plain line tracks can cause or contribute to a derailment:
- CWR buckling
- Incorrect crosslevel
- Incorrent cant/superelevation
- Incorrect alignment
Track-caused derailments are often caused by wide gauge. Proper gauge, the distance between rails, is 1,435 mm (4 ft 8 1⁄2 in) on standard gauge track. As tracks wear from train traffic, the rails can develop a wear pattern that is somewhat uneven. Uneven wear in the tracks can result in periodic oscillations in the truck, called truck hunting, which can be a contributing cause of derailments.
In addition to rail wear, wooden ties can weaken and crack from the stress of bearing train load tonnage. As ties weaken, they lose a solid tight grip on the spikes, which hold the rails in position. Over time, the rail gauge can drift substantially from the proper specification, hence the need for regular track maintenance and tamping. More usually, a rail that is not properly held in position tends to roll when a train passes over it at excessive speeds. In that case, poorly maintained track and excessive speeds are both contributing causes for the derailment.
Train tracks most often lose gauge in curves, where the outside wheels tend to push the gauge rail outward. If the gauge between the rails are sufficiently wide, the train wheels can drop between the rails. This, however, is not a common cause of derailments.
Many rail operators in the United States are replacing wood ties with concrete ties on lines with heavy or high speed trains. Amtrak's Acela New Haven to Boston Electrification Project replaced practically all wooden ties between New Haven and Boston with concrete ties. However, converting existing tracks to concrete ties is a costly and time-consuming method to reduce out-of-gauge derailments.
Concrete ties have been standard on mainline railroads in Europe since the 1960s. Concrete ties have also been the renewal standard on rapid transit applications in North America. For subway tunnels, 'slab track' is the preferred option, where support structures for rails are directly poured into the tunnel floor using pre-mixed concrete.
Excessive speed derailments
Two different mechanisms cause excessive speed derailments:
- Wheel climb, in which the wheel is lifted off the track because the friction between the flange and the gauge face of the rail is too great, causing the wheel flange to climb outwards over the head of the rail.
- Rail roll, in which the horizontal forces applied by the flange to the gauge face of the rail is too great, overcoming the anchoring forces of rail spikes and clips.
These are two extreme conditions that result from excessive vehicle speed. The "L/V ratio," which is the ratio of the lateral to vertical forces on the rail, is a critical factor in maintaining a safe speed.
In the United States, the maximum permissible speed for set degree of curvature and superelevation is defined in 49 CFR, Part 213. In the UK, the Rail Group Standards defines maximum permissible speeds.
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.
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.
Several types of derailments can be caused by in-train forces.
- Uneven loading
- Train "stringlining" on sharp reverse curves
- Poor train handling techniques
- Rolling stock design issues
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 causes couplers to overload.
This type of derailment occurs when a string of light cars travel over 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 chord to the arc.
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.
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. 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.
A similar 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.
Flangeless wheels make it easier for a locomotive to negotiate curves, but make them more prone to derailment. Rerailing a train after it has derailed is not an easy task, and often requires the use of large rail-mounted cranes.
The Australian Standard Garratt of WWII had flangeless driving wheels which made it derailment prone.
Wheel and truck failures
Wheel fracture derailments are quite rare. This is partly due to the Federal Railroad Administration's requirement for 1,000-mile (1,600 km) undercarriage inspections for trains operating in the U.S. Also, a variety of defect detectors en route would highlight 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 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 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, rather than allowing trains to continue running and potentially hitting a deformed structure or track segment.
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 U.S., minor low speed derailments are sometimes rerailed by the engine crew. Wooden blocks, planks, 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.
In some cases, cars are simply left in the field after the derailment, because the cost of retrival 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 U.S., smaller railroads often rely on external contractors for disaster recovery.
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.
George Westinghouse, amongst others, invented devices that helped rerail derailed vehicles.
Most railway accidents involve derailment. See Lists of rail accidents.
- November 11, 1833 – Hightstown, New Jersey, United States: 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.
- 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.
- Zuerich-Affoltern 1994 - 5 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.
- 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.
- April, 2008 - Larissa, Greece - passenger train derails; 28 of 174 passengers injured
- 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 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.
- Lists of rail accidents
- Classification of railway accidents
- Train wreck
- Tram accident
- Kenya Railways Corporation - accidents
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