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.