On the railway line between Zutphen and Hengelo in the Netherlands, rolling contact fatigue (RCF) defects are found at the surface of the rail. RCF is a known hazard to the safe operation of railways, the cracks can cause transverse rail fracture, which may result in derailment. Contrary to convention, the traffic on this particular railway is bidirectional. In literature, little is known about the effects of bidirectional traffic on the rail material and the formation of RCF defects. In this study, the effects of the bidirectional traffic on the rail material and RCF defects are investigated by performing a metallographic examination of the rail surface and microstructure of rail material specimens taken from the actual track. The metallographic examination is supported by the analysis of the wheel-rail contact, which is simulated with multibody wheel-rail contact simulation software.

In longitudinal direction, optical microscopy revealed the absence of plastic strain in the surface layer of the rail. This plastic strain, which is the result of tangential surface traction, is always observed on rails subjected to uni-directional traffic and is considered the leading cause of fatigue crack initiation. However, the simulated wheel-rail contact stresses exceed the yield limit, so plastic deformation is expected. It is concluded that the frequent direction reversal of the tangential surface traction causes cyclic plasticity without the accumulation of strain, detectable with optical microscopy. Despite the absence of accumulated plastic strain in longitudinal direction, RCF cracks of varying sizes are observed on the rail surface. Although developed as a result of bidirectional traffic, the observed crack morphology shows similarities to the 'squat' RCF defect.

In this study, new insights are generated by considering the implications of bidirectional traffic and the rolling direction reversal on existing theories about the rail squat formation mechanism. Directions are formulated for further research on cyclic plasticity without the accumulation of plastic strain.