Cementite decomposition leading to the formation of the white
etching layer (WEL) on the surface of rail tracks was investigated by
electron energy loss spectroscopy (EELS) in the TEM and by threedimensional
atom probe (3D-AP). It is shown that the carbon
concentration inside cementite laths undergoing severe plastic
deformation in the transition zone between the pearlitic substrate
and the WEL remains stoechiometric. However there exists a boundary
layer at the ferrite/cementite interface where the carbon composition
decreases, indicating that decomposition is an interfacial
phenomenon. Carbon atoms diffuse into the ferrite and segregate
along structural defects leading to a heterogeneous supersaturated
composition in the white etching layer.