Tonal squeal noise (i.e. the high-amplitude singing of a railway wheel with pure tone components) is emitted by some trailing inner wagon wheels on heavy haul trains traversing 1000 m radius curves on the iron ore export line in South Africa. Field measurements have shown that the trailing inner wheels that squeal are subject to predominantly longitudinal creepage with little-to-no lateral creepage. The longitudinal creepage acting at the contact of the squealing wheels exceeds 1%, which supports the likelihood of creep saturation and subsequent squealing due to unsteady longitudinal creepage in the large-radius curves. Experimental modal analysis of the wheel types identified to be relevant to squeal has revealed that for each unstable frequency, two eigenmodes are likely to be important: one that has a large mode shape component at the wheel–rail contact in the circumferential direction and another that has a large mode shape component at the wheel–rail contact in the radial direction. A frictional self-excitation mechanism based on mode-coupling is favoured as being responsible for squeal excited in large-radius curves.