The adhesion force between rail and wheel is one of the most important factors for operating high speed railways, whereas, it is anticipated that the water film formed between rail and wheel has remarkable influence on the adhesion force under rainy conditions at high speeds. In this study, we investigated the influence of important factors such as rolling speed, contact pressure and water temperature on the water film thickness for smooth surfaces by applying the EHL theory. We also developed an empirical equation for the water film thickness related to rolling speed, loads and material parameters based on numerical solutions by using the linear regression method. In order to investigated the influence of surface roughness on the adhesion force, we further calculated adhesion coefficients with a simplified calculating method and compared the theoretical results with the measurements obtained from the filed tests with Japanese Shinkansen vehicles.