A steam locomotive's driving wheels and valve gear.
A driving wheel is a powered wheel on a steam locomotive, which is driven by the locomotive's pistons and coupling rods (or turbine, in the case of a steam turbine locomotive).
On diesel and electric locomotives, the driving wheels may be directly driven by the traction motors (electric motors). Coupling rods are not usually used, and it is quite common for each axle to have its own motor. Jackshaft drive and coupling rods were used in the past (e.g. in the Swiss Crocodile locomotive) but their use is now confined to shunting locomotives.
On an articulated locomotive or a duplex locomotive driving wheels are grouped into sets which are linked together within the set.
An example of how driving wheels move.