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.
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.
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.
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.
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.
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.
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.
Several different types of misaligned plain line tracks can cause or contribute to a derailment:
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.
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 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:
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.
AT SCHUMER'S URGING, FEDERAL RAILROAD ADMIN. TO INVESTIGATE NORFOLK SOUTHERN'S TRAIN DERAILMENT THAT LEFT TRAIN CAR OVERTURNED IN ELMIRA PARKING LOT-WITHOUT LOCAL OFFICIALS' NOTIFICATION EXAMINATION MARKS STEP TOWARDS STRONGER REPORTING REQUIREMENTS WHEN TRAINS GO OFF THE TRACK FRA WILL ALSO INVESTIGATE REPORTING REQUIREMENTS IN NEW YORK STATE IN RELATION TO THE ELMIRA DERAILMENT; SCHUMER WI.
Mar 06, 2012; WASHINGTON -- The following information was released by New York Senator Charles Schumer: Today, U.S. Senator Charles E. Schumer...