scope:

This Track Work Information Sheet covers the management of cyclic irregularities. The most common form of cyclic irregularity is cyclic top, but cyclic crosslevel and alignment are also possible. These types of irregularities are caused when the natural harmonics in the vehicles are excited by track faults (often quite minor track faults) causing the vehicles to bounce or sway. The faults in the track appear as a series of evenly spaced waves in the track, in the form of dips or lateral misalignments. These evenly spaced irregularities then grow in magnitude and number as traffic continues to pass over the area.

Track recording vehicles will identify sites of cyclic top and give the severity and extent of the problem. However, cyclic crosslevel and alignment are not readily picked up from the recording traces but can usually be clearly identified on site. Early action is essential as deterioration can be rapid and extensive.

Determining the cause can be difficult as cyclic top grows from an initial trigger and propagates in dip severity & dip numbers with the passage of traffic.

Implementing a remedy can be quite straightforward provided the ballast is of good angular quality, able to drain freely & no underlying formation problems. If this is not the case, then standard remedies will only provide a temporary rectification before the cyclic top returns over continued passage of traffic.

See also separate instructions:

TWI 2G004 How to measure voids

TWI 2G019 How to apply a speed restriction in an emergency

TWI 2L001 How to clean a ballast shoulder

TWI 2P035 How to maintain steel sleeper track

TWI 2T008 How to prepare track for tamping

TWI 2T009 How to recognise cyclic top

TWI 2T010 How to carry out Kango packing

TWI 2T010a How to carry out Orbital tamper packing

TWI 2T020 How to look after track after lifting & packing or tamping

TWI 2T026 How to repair a level 2

TWI 3B006 How to manage multiple wet bed formation

TWI 3G093 How to remove an emergency speed restriction

TWI 3T020 How to plan a DTS TWI 3T045 How to recognise ballast memory