The polygonal wear of train wheel commonly occurs on railway transport and it causes strong vibration and noise in railway vehicles and tracks. Serious polygonal wear can threaten the running safety of the train. In this paper, more than thousands sets of polygonal wheels are tested and the test data are statistically analyzed to find out the present status and the features of the polygonal wear of train wheels. As an example, theoretical analysis and experimental study are used to find out the root cause of 18th~20th order polygonal wear. The results show that the root cause of polygonal wear is in connection with the high frequency (580 Hz) flexible resonance of the wheel-rail system. The high frequency (580 Hz) flexible resonance of the wheel-rail system occurs under the fierce excitation of wheel/rail irregularities. The work can be utilized to prevent polygonal wheels occurrence and to provide references for further study.