1.3 Thermal damage
Although there is always some heat dissipated at the wheel/rail contact, this is rarely sufficient to cause a change in the microstructure or internal stresses in a rail. Such changes do, however, occur when wheels slip sufficiently.
On rails that have been in service for a sufficiently long time, there is often one band or more of so-called “white phase” or “white etching layer” (WEL). These bands of WEL are extremely shallow (about 0.050mm deep) and comprise martensitic material that has resulted from many small wheelslip events, each of which is sufficient to heat the rail locally to the austenitizing temperature. The rapid quenching that arises from the rail’s high thermal inertia causes the transformation to martensite.
There are many cracks in the layer of WEL itself, but these cause the layer to detach; they do not propagate into the rail. The detached particles of WEL are essentially wear debris.
There is evidence that WEL has become less common with AC traction, probably as a result of better intrinsic wheelslip control.