Analysis of Differences in Traction and Braking Wheel–Rail Adhesion Characteristics under Conditions of Third-Body Contamination

Train idling in traction and skidding in braking pose significant risks to operational safety. The traction and braking performance of rail vehicles is constrained by the wheel-rail adhesion state. Optimizing the target acceleration/deceleration design values and adhesion control strategies based on wheel-rail adhesion characteristics can enhance adhesion utilization and ensure train operational safety. Current research has primarily investigated wheel-rail adhesion characteristics with third bodies under either traction or braking conditions separately, without systematic comparative analysis of their differences. This limitation hinders further optimization of adhesion control strategies for both traction and braking. To systematically investigate and compare potential differences in wheel-rail adhesion characteristics between traction and braking under conditions of third-body contamination, this paper performed wheel-rail adhesion characteristic tests under various conditions. These conditions included different speeds (20–80 km/h), axle loads (11–17 t), and progressively increasing maximum slip ratios (with the maximum slip ratio incrementally raised until the adhesion improvement point emerged) with water and anti-wear fluid as third bodies. The results indicate a significant discrepancy in the slip ratio conditions required for adhesion improvement on the adhesion coefficient-slip ratio curve between traction and braking conditions: a higher slip ratio is required for adhesion improvement under traction conditions compared to braking conditions. Ultimately, this paper elucidates and validates the underlying mechanisms by which distinct wheel-rail contact temperatures under traction and braking conditions give rise to differences in adhesion characteristics.