When two surfaces move against each other, frictional force is generated. If friction is low, movement is easy, and if it is high, it becomes more difficult. Perhaps the most crucial difference between rails and roads is the level of friction. Movement is smoother on tracks.

We greatly benefit from the low friction that naturally occurs between hard wheels and a smooth rail. This is because the contact area between the materials is minimal. Therefore, it rolls so easily on tracks. In the case of a car wheel, the contact area with the road is several times larger. The car needs high friction against the road to be able to steer; otherwise, it would soon end up in a ditch.

On rails, the wheels follow the twists and turns of the track and do not require friction for steering. At the same time, it is the wheel's friction against the rail that allows it to accelerate or brake. Especially in uphill and downhill sections, this becomes a problem. Therefore, the challenge has always been to lay the track with as little slope as possible.