scope:

Introduction

This Track Work Instruction covers how to recognise possible cyclic top sites.

Cyclic top is a series of evenly spaced dips in the track that can be a problem to certain types of rail vehicles at particular speeds.

What separates this defect from normal poor top is that it can derail certain types of rail vehicles, even though each top defect may not look that severe.

Derailments happen when a vehicle begins to bounce up and down and at the same time starts to roll from side to side. It will also yaw from end to end. The whole process can develop very quickly and is increasingly likely to occur as speeds increase over 30mph. The motion becomes violent enough for one wheel to become very lightly loaded.

With the yawing of the vehicle there is everything needed for flange-climb and thus a derailment.

Most cyclic top derailments occur on curved track although they can and do happen on straight track.

It is the vertical movements of the vehicle, combined with the yawing motion, that cause cyclic top derailments on straight track.

The number of defects needed to cause a derailment can be as few as two!

In identifying cyclic top sites many other non-risk sites will be picked up in the process.

Any site with poor top may be cyclic but it can be difficult to see this by eye. If in doubt, report back.

Individual defects may not be severe - it is the regular spacing of the defects that can lead to the creation of a problem.

Throughout the defective stretch of track there may be no serious individual defects but there will be a series of evenly spaced relatively minor ones.

As a typical example, a recent derailment site had no level 2 top defects but had 23 level 1 top defects (or roughly 1 every 30 ft).

See also separate instruction

TWI 3T028 How to manage cyclic top