Keywords: wheel-rail interface, friction, humidity, oxidation, water contamination, squealing.

Environmental moisture strongly affects wheel–rail friction and friction states relevant to curve noise in urban rail systems. Although increasing relative humidity is generally expected to reduce adhesion, field observations indicate that humid periods may be associated with friction conditions that lead to increased curve noise. This paper investigates the underlying contradiction through laboratory experiments on a rail segment using a portable rail tribometer, a controlled climatic chamber, and a rolling–sliding contact simulator. The results confirm the classical short-term effect of humidity, with decreased adhesion, while prolonged exposure to high humidity or direct water exposure leads to a delayed increase in dry adhesion due to oxide and oxyhydroxide layer formation. Subsequent dry wheel passages further increase adhesion by mechanically conditioning the running band. The study demonstrates that the final friction state is governed by the coupling between moisture-driven tribochemical surface preparation and subsequent traffic-induced mechanical activation.

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