A field investigation was conducted on a heavy haul railway line with axle load of 25 t. There was a significant difference in rail rolling contact fatigue (RCF) on the entering and leaving transition sections on curves with radius of 580?1000 m, and particularly, the RCF on the leaving transition section was severer. A dynamic model of heavy haul train including two locomotives and 108 wagons was established using Simpack on the basis of on-site wheel/rail observation and train parameter investigation, and a damage function model was applied to numerically analyze the mechanism of the rail RCF difference on the entering and leaving transition sections. Result shows that the RCF difference is dominated by the curving behavior of wagons, and the contribution of the leading wheelsets is the most significant, while the contribution of the trailing wheelsets and locomotives is relatively slight. More detailed analysis shows that the RCF is not significant under the condition of standard wheel/rail profile matching. However, after the wagon wheels wear, the wheel/rail creepage and creep force on the leaving transition section are higher than those on the entering transition section, which is the primary reason for the RCF. And the effect of rail worn profile is not significant for the RCF. The frequent interaction between the worn wagons leading wheelsets and the worn rail on the sharp radius curve is the main reason for the rail RCF difference on the entering and leaving transition sections.