Using the methods of transmission electron microscopy,the carbide phase evolution in surface layers of the differentially quenched rails is studied after the passed tonnage of 691.8 million tons at the depth up to 10 mm along the central axis and fillet of rail head.The action of two mutual supplement mechanisms of steel carbide phase transformation in surface layers at rail operation is established:(1) cutting mechanism of cementite particles with the following departure in the volume of ferrite grains or plates (in pearlite structure);(2) cutting mechanism and following dissolution of cementite particles,transfer of carbon atoms on dislocations (in Cottrell atmospheres and dislocation cores),transfer of carbon atoms by moving dislocations into ferrite grains volume (or plates) with the following repeated formation of nanosized cementite particles.The first mechanism is accompanied by the change in linear sizes and morphology of carbide particles.Cementite element composition change is not essential.Carbide structure change can take place during the second mechanism.