The term adhesion railway or adhesion traction describes the most common type of railway, where power is applied by driving some or all of the wheel

Wheel

A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

s of the locomotive. Thus, it relies on the friction

Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

 between a steel wheel and a steel rail. Note that steam locomotives of old were driven only by 'drivers' that were connected to the 'side rods' of the locomotive. These were the large wheels. On diesels, traction electric motors (DC or AC) drive all or some of the axles.