Normally you do both: The running surface of the wheels is sloped, but the rails are also mounted onto the sleepers with a corresponding inclination. (Some groo

Normally you do both: The running surface of the wheels is sloped, but the rails are also mounted onto the sleepers with a corresponding inclination. (Some grooved rail profiles for streetcars/trams have the inclination built directly into the profile of the railhead, so the rail itself is mounted upright, but you get the same effect at the railhead level.)