Experimental study on the effect of liquid media and sanding particles on wheel-rail adhesion behavior under track ramp conditions

As high-speed railways expand into steep mountainous regions, the wheel-rail system must not only address low adhesion caused by liquid media (water and oil) in an open environment but also ensure traction and braking capability on steep gradients. Sanding is an effective method to improve wheel-rail adhesion. This study examined the effect of liquid media and sanding particles on wheel-rail adhesion behavior under track ramp conditions using a novel 1:5 scale wheel-rail rolling contact experimental machine. The results indicated that, under dry and water conditions, with the increase in track gradient, the adhesion coefficient decreased. However, under oil conditions, the track gradient had little influence on the adhesion coefficient. At the same track gradient, the adhesion coefficient on the ascent was higher than on the descent conditions. Furthermore, sanding notably increased the adhesion coefficient under low adhesion conditions, but the adhesion improvement effect was affected by the particle distribution density, size, and type. Specifically, under water conditions, at a 0% track gradient, with the increase in the particle distribution density, the adhesion coefficient increased, but it decreased when the distribution density exceeded a threshold; Smaller particle size silica sand showed the best adhesion improvement. When the track gradient increased to 5%, the influence trend of the particle distribution density and particle size on adhesion coefficient under water and oil conditions was consistent with that at 0% track gradient. Furthermore, random forest importance analysis indicated that particle distribution density should be prioritized for adhesion improvement under track ramp conditions when water or oil is present at the wheel–rail interface. Within the tested range of the present 1:5 scaled experiments, S-sized silica sand at 0.2 g/m under water conditions and M-sized silica sand at 1.0 g/m under oil conditions showed better adhesion improvement performance.