Top-of-Rail Friction Modifiers for Wheel–Rail Adhesion Control: Roles of Application Dosage, Viscosity, and Rheological Index

Top-of-rail friction modifiers (TORFMs) act as transient interfacial films that regulate wheel–rail adhesion under rolling–sliding contact. Unlike permanent coatings, these films undergo continuous formation, depletion, and regeneration during repeated wheel passages. This study investigates TORFM-treated interfaces by combining laboratory rolling-contact tests with a starved non-Newtonian elastohydrodynamic lubrication (EHL) model. Experimentally, application dosage and viscosity were varied on a CL60 wheel–U71Mn rail pair, and the adhesion coefficient was measured as the validation quantity. Numerically, a model-based sensitivity analysis was used to examine how dosage-derived effective supply thickness, viscosity, and rheological index influence film formation and load sharing. The experiments show that increasing dosage alleviates inlet starvation and lowers initial adhesion, which then recovers toward dry-contact levels during repeated rolling. Increasing viscosity from 0.002 to 0.022 Pa·s reduced the minimum adhesion coefficient from 0.21 to 0.086, although excessive viscosity may prolong the low-adhesion stage. The rheological-index calculations (n = 0.7–1.0 around the measured formulation value n = 0.82) indicate that shear-responsive flowability can affect contact-zone supply and the film-supported load fraction; these rheological-index trends are model-based sensitivities rather than independently validated experimental effects. Overall, the results support a transient interfacial-film interpretation of TORFM-controlled adhesion and suggest that practical design should target a controllable medium-adhesion window (approximately 0.15–0.30 under the present laboratory conditions) rather than minimum friction alone.