A series of experiments on the wheel/rail traction coefficient were carried out with a twin disc rolling contact machine by varying the surface roughness and the temperature of the wheel and rail discs, and using sprayed water, for the purpose of obtaining fundamental knowledge about how to prevent wheels slipping during driving and wheel sliding during braking. The results showed that the maximum traction coefficient value occurred in the range of 1 to 3 μm of the combined roughness under any temperature conditions, and its value increased to 0.4 with increases in temperature.