Friction acts to prevent or resist relative motion between the two surfaces. So, if there is a torque on the wheels and the point of contact can not move relative the rail (just where it touches) because of static friction, the only way the wheel can turn is if the train moves relative the wheel.

Friction is the "invisible force" that allows a train to move, steer, and stop. It is most critical in the form of adhesion—the friction between the steel wheels and the steel rails. Because steel on steel has a very low coefficient of friction (smooth on smooth), it is highly efficient for cruising but makes starting and stopping difficult. High friction is necessary for traction; if the rails are "greasy" due to rain or crushed leaves, the wheels will slip (spin in place), preventing the train from accelerating. Conversely, friction is the primary mechanism for braking. Traditional tread brakes apply friction directly to the wheel surface, while disc brakes use pads. However, if friction is too low (low adhesion), the wheels can lock and slide, causing "flat spots" on the wheels that require expensive repairs. To manage this in 2026, modern trains use "sanding" systems, which drop grit in front of the wheels to artificially increase friction when the system detects a loss of grip.