ABSTRACTSevere vibrations and an increase in structural component fatigue failures have been observed on a freight locomotive class operating on a heavy haul route within South Africa. The root cause of these severe vibrations were investigated experimentally and it was found that polygonised wheels were cuasing these severe vibrations. A coupled mode of vibration exists in the traction system that is a combination of traction motor pitching and wheelset axle torsion. Harmonics and interharmonics are features of an AC motor controlled by a variable frequency drive (VFD) and were confirmed to cause resonance of the coupled vibration mode. Interharmonics are a function of the fundamental frequency () of the VFD and the input AC frequency ( supplied to the VFD. It was found by experimentation that the interharmonic caused resonance of the coupled mode while the locomotive operated at 40 km/h. This running speed and frequency of vibration was found to fix the wavelength of a 20th order polygon on the wheels during approximately 11% of the locomotive’s load hauling time.