As far as the damage behavior of materials is concerned, the
crack of wheel material initiated at the center of the contact zone and propagated tangentially along the sub-surface with the tendency toward to the bulk (as shown in Fig. 12a). Fig. 12b shows a cross-sectional morphology of the worn rail specimen. The sub- surface microstructure became loose and fragmental accompanying an increase of the material volume, the crack was initiated and then propagated toward the surface; finally the delamination of