Abstract: Instruments such as stereo microscope, optical microscope and scanning electron microscope were used to analyzing the falling weight fracture reason of PG4 heat treatment rails. The results show that when falling weight test, all rails fractured from the arc transition area of the rail base where there was a layer of martensite microstructure on the surface. During the rail transporting process, glide friction was formed between the rail base angle and the rim root of the transporting roller, and the friction heat resulted in the temperature instantaneous rising to the austenitizing temperature of the rail metal and formation of hard and brittle martensite microstructure after subsequent quick cooling. So the rail fractured only for a hammer under the condition of hammer mass of 1 000 kg and hammer height of 6.4 m.