3.1 “P2 resonance” corrugation
The so-called “P2 resonance” of the vehicle’s unsprung mass bouncing on the track stiffness is a common wavelength-fixing mechanism on all types of track, from trams to heavy haul railways. The examples shown are for a tram track in the street, a mass transit system, a freight railway, and a mixed passenger and freight line. Wear is the “damage mechanism” for the first two of these and plastic deformation for the others.
P2 resonance corrugation is a problem for metros in particular because it gives rise to low frequency “rumble” that is transmitted well into buildings. For conventional railways the wavelength of the resulting corrugation is usually at least several hundred millimetres, making the corrugation difficult to see.
Low rail corrugation on the mixed passenger and freight line shown has occurred because the line is canted to suit passenger trains, and over-canted for the very much slower freight trains. Bogies steer badly in these conditions, giving very high tangential loading on the low rail. This causes quasi-continuous plastic flow, on which corrugation is superposed.
P2 resonance corrugation on non-ballasted track can usually be avoided using a sufficiently resilient, well-designed trackform.
A proven treatment of corrugation is to use harder rails as these reduce damage by both plastic flow and wear. Reprofiling should be undertaken routinely to retain an appropriate transverse profile that does not cause high stresses and RCF damage, and to minimise irregularities that “trigger” dynamic behaviour.
An occasional damage mechanism is yield of the rail in flexure. When this is the case the corrugation can be avoided using rail with higher flexural rigidity and strength. Dynamic loads are usually critical, so irregularities should be minimised.