Cyclic Top occurs when a dip in a rail causes the suspension of a vehicle passing over it to bounce. The track at the end of the bounce then receives an impact loading that can create a second rail dip and, as that second dip deteriorates with successive impacts, trains will then bounce a second time, creating a third dip. After time a sequence of dips can be created which will, depending on the suspension characteristcs of the trains using the line, cause each successive bounce to increase to the point where the vehicle will derail. This very much depends on the “natural resonance frequency” of the vehicles. As the vehicles most susceptable to this are freight wagons of various types, the extra damping on passenger suspension tending to act against this, a speed limit of 30mph is placed on known cyclic top faults for freight trains to prevent the bouncing building up to the derailment point and also to reduce the rate at which the track is deteriorating. The fault then is recorded for rectification within a timescale that depends on the fault magnitude, with severe faults being repaired within 36 hours.
