For conventional railways, the traction and braking forces are transmitted from the train to the track by means of the frictional adhesion between the wheel and the rail. If the forces are greater than the available adhesion force, the train will slip. See: Adhesion
A measurement of the force required to remove a tape from a substrate.
The amount of pulling or retarding force a wheel is capable of exerting on the rail without slipping or spinning. The base coefficient of friction between the wheel and the rail are the determining factors. These factors are affected by the load on the wheel, wheel and rail condition and external factors such as oil, rust & weather.