The heavy-haul railway for 40 t axle load freight is one of key development aspects. It is the premise of heavy axle load technology to obtain wheel/rail interaction features based on the analysis of impact vibration. The train and track coupling dynamic model of freight with 40 t axle load is put forward. The wheel flat is simulated by varied wheel rolling radius. The results show that the wheel flat leads to severe wheel/rail force，and impact energy with high frequency region. The critical speed of wheel flat increases with the growth of wheel flat length. The critical speeds of wheel flat with length of 10，20，30 and 40 mm are respectively 30，50，70 and 80 km/h. The critical speed of wheel flat exceeds 100 km/h when wheel flat length is more than 50 mm. The wheel/rail force with higher frequency occurs on wheel/rail interface，and cannot cause the rail deformation adequately. The wheel/rail force in region of less than 850 Hz can be applied on the track，which should be paid attention during track component design.The suggestive critical length of the wheel flat should be less than 40 mm according to the sleeper design strength requirement.