Editing Derailment (section)

=== Broken rails ===
Broken rails are a leading cause of derailments. According to data from the Federal Railroad Administration, broken rails and welds are the most common reason for train derailments, making up more than 15 percent of derailment cases.<ref>{{Cite web |last=Tracy |first=Abigail |last2=Reznik |first2=Tal |last3=Volcativ |title=Broken Rails Are Leading Cause of Train Derailments |url=https://www.scientificamerican.com/article/broken-rails-are-leading-cause-of-train-derailments/ |access-date=2022-12-01 |website=Scientific American |language=en}}</ref>[[File:Schienenbruch.jpg|thumb|A broken rail, probably starting from [[hydrogen embrittlement|hydrogen inclusion]] in the rail head]]
A traditional track structure consists of two rails, fixed at a designated distance apart (known as the [[track gauge]]), and supported on transverse sleepers (ties). Some advanced track structures support the rails on a concrete or asphalt slab. The running surface of the rails is required to be practically continuous and of the proper geometrical layout.

In the event of a '''broken or cracked rail''', the rail running surface may be disrupted if a piece has fallen out, or become lodged in an incorrect location, or if a large gap between the remaining rail sections arises. 170 broken (not cracked) rails were reported on Network Rail in the UK in 2008, down from a peak of 988 in 1998/1999.

* In [[jointed track]], rails are usually connected with bolted [[fishplate]]s (joint bars). The web of the rail experiences large [[shear force]]s and these are enhanced around the bolt hole. Where track maintenance is poor, [[Fatigue (material)|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.
* 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 embrittlement|hydrogen inclusion]] during the manufacturing process, leading to [[crack propagation]] under fatigue loading.
* Local embrittlement of the parent metal may take place due to wheel spin (traction units rotating driving wheels without movement along the track).
* Rail welds (where rail sections are joined by welding) may fail due to poor workmanship; this may be triggered by extremely cold weather or improper stressing of continuously welded rails, such that high tensile forces are generated in the rails.
* The fishplates (joint bars) in jointed track may fail, allowing the rails to pull apart in extremely cold weather; this is usually associated with uncorrected rail creep.

Derailment may take place due to excessive '''gauge widening''' (sometimes known as '''road spread'''), in which the sleepers or other fastenings fail to maintain the proper gauge. In lightly engineered track where rails are spiked (dogged) to timber sleepers, spike hold failure may result in rotation outwards of a rail, usually under the aggravating action of crabbing of bogies (trucks) on curves.<ref name="wu"/>

The mechanism of gauge widening is usually gradual and relatively slow, but if it is undetected, the final failure often takes place under the effect of some additional factor, such as excess speed, poorly maintained running gear on a vehicle, misalignment of rails, and extreme traction effects (such as high propelling forces). The crabbing effect referred to above is more marked in dry conditions, when the coefficient of friction at the wheel to rail interface is high.