Keywords: torque-modulation, phase difference, Coefficient of friction, experiment, V-Track, creep-curve.

Train-borne measurements of the Coefficient of Friction (CoF) through wheel slip can induce wheel-rail interface damage, and indirect filtering methods exhibit limited universal applicability due to their dependence on parametric models. To overcome these limitations, a torque-modulation-based CoF measurement method has been proposed, in which small sinusoidal torque modulations are superimposed on wheelset torque, and the resulting phase difference between the applied torque and wheel angular velocity signals indicates the CoF at the wheel-rail interface in proximity to friction saturation. The concept has been tested and verified by multibody dynamics simulations, whereas experimental validation has not been reported in accessible literature. This study experimentally validated the torque-modulation method using the V-Track test rig in the lab conditions. Torque modulation was systematically performed on the V-Track at multiple operating points along the creep curve, from low-torque conditions to friction saturation. At each operating condition, the applied torque and wheel angular velocity were recorded over three modulation cycles. Detailed data-processing procedures for phase-difference extraction are presented. The test results demonstrate good repeatability of phase difference over cycles. The measured phase-difference behaviour shows good agreement with theoretical predictions and simulation trends, confirming the fundamental validity of the torque-modulation method for train-borne CoF measurements.

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