Low Adhesion Due to the Wet-Rail Phenomenon: Influence of Particle–Fluid Interaction in Wheel–Rail Contact

The wet-rail phenomenon can cause low adhesion, which negatively affects railway operation. It is believed to occur when small amounts of water mix with solid particles on wheel and rail surfaces, e.g., wear debris or iron oxides, forming a dense suspension in the wheel–rail contact, leading to sharp adhesion drops. Mini Traction Machine (MTM) tests using water-based suspensions with different particles also show adhesion drops during water evaporation, which can be linked to the wet-rail phenomenon. While the physical mechanisms underlying the adhesion drop are unclear, it is hypothesised that rapid loading raises fluid pressure in the suspension, separating wheel and rail surfaces, reducing force transfer through particle contact, thereby reducing the suspension’s shear strength. For verification, a coupled Discrete Element Method and fluid dynamics model is used to simulate a simplified MTM setting and steps towards full scale wheel–rail contact. During simulation of rapid loading, fluid pressure rises but remains negligible compared to applied contact stresses in all considered cases. Thus, it is unlikely that hydrodynamic pressure build-up within the suspension contributes significantly to the low adhesion observed. Future research should investigate additional mechanisms, such as reduced shear strength of deformed or crushed wet particles under high normal loading conditions.