Wheel/rail adhesion: its relation to rail contamination on british railways

An investigation has been made how wheel/rail adhesion varies and how this may be related to railhead contamination. Experiments with oils and water deposited on rails have been carried out. In dry weather adhesion depends on the quantity of oil present, which normally is so small it affects friction by a boundary lubrication mechanism. Evidence suggests that water reduces adhesion from two causes. On debris free rails it acts as an additional boundary lubricant. With debris present it forms a low shear strength mixture which, in minimally wet conditions, remains on the wear band where wheels contact the rail. In steady rain the debris mixture is squeezed aside and adhesion is possibly improved. In dry weather, debris particles have apparently little influence on the overall adhesion coefficient.