A derailment is an accident on a railway in which a train leaves the rails, which can result in damage, injury, and 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.
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.
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 aren't 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 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 susceptable to brittle fracture rather than plastic deformation. It is therefore imperative that unintentional impurities in rail be minimized. Corus of Holland and England, and U.S. Steel of Pittsburgh, are two current rail manufacturers.
Wheelburn-related rail breaks
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 no larger than a dime in size, 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 susceptable to breakage is created.
Wheelflat-related rail breaks
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.
Cold weather-related rail breaks
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 56.5 inches (four feet, eight-and-a-half inches) 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.' Truck hunting 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 loose 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 isn't 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 high tonnage 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 readymix 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.
Wheel and truck failures
Wheel fracture derailments are quite rare. This is partly due to the Federal Railroad Administration's requirement for 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 + 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.
Most railway accidents involve derailment. See list of rail accidents.
- November 11, 1833 – Hightstown, New Jersey, United States: Carriages of a Camden & Amboy train derail at 25 miles per hour 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 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, West Virginia, United States: Two Baltimore & Ohio passenger cars tumble down a 100 foot ravine above the Cheat River in West Virginia, west of Cumberland, Maryland, after they are derailed by a loose rail.
- December 12, 1917 – Saint Michel de Maurienne, France: A military train derails at the entrance of the Fréjus Rail Tunnel after running away down a steep gradient; 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 speeds through an open switch. Four passengers, the engineer, fireman and conductor are killed.
- 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.
- 2000 – Hatfield rail crash.
- May 10, 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.
- February 23, 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.