On a steam locomotive, a wheel which is driven by the locomotive's pistons (or turbine, in the case of a steam turbine locomotive). On a conventional, non-articulated locomotive, the driving wheels are all coupled together with side rods (also known as coupling rods); normally one pair is directly driven by the main rod (or connecting rod) which is connected to the end of the piston rod; power is transmitted to the others through the side rods.
On an articulated locomotive or a rigid-framed locomotive with divided drive, such as a Duplex locomotive, driving wheels are grouped into sets which are linked together within the set.
Driving wheels are generally larger than leading or trailing wheels. Since a conventional steam locomotive is directly driven, one of the few ways to 'gear' a locomotive for a particular performance goal is to size the driving wheels appropriately. Freight locomotives generally had driving wheels between 40" and 60" in diameter; dual-purpose locomotives generally between 60" and 70", and passenger locomotives betwen 70" and 100" or so.