Derailment - Misaligned Railroad Tracks

Misaligned Railroad Tracks

Several different types of misaligned plain line tracks can cause or contribute to a derailment:

• Wide gauge
• Buckling
• Incorrect track geometry
• Washout
• Dynamic effects
• Rail profile problems
• Rail rotation

Wide gauge derailments occur when the rails are allowed to be significantly wider than the proper gauge under load, resulting in one or more wheelsets dropping in to 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.

Buckling 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 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 effects 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 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.

Rail profile 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.

Rail rotation can take place when inadequate holding-down forces are available, allowing the rail to rotate outward, resulting in a gross increase in gauge. This is most likely in very low-grade track when very poor sleeper quality is present.