The carrying wheels on a steam locomotive are those wheels that are not driven, i.e. they are uncoupled and run freely, unlike coupled or driving wheels. They are also described as running wheels[1] and their axle may be called a carrying axle. Carrying wheels are referred to as leading wheels if they are at the front, or trailing wheels if they are at the rear of the locomotive. See main article Bogie.
In particular reference to steam engines, the carrying wheels have a very important purpose of allowing the engine's weight distribution to be altered. For example in the use of leading wheels it would allow the boiler to located further forward of the driving wheels, the weight of which counters the leverage imposed by the drawbar and the load of the pulled wagons/cars about the fulcrum of the rearmost driving wheel. Similarly the trailing wheels can move the fulcrum to the rearmost trailing wheel. Such change can dramatically improve the operating speeds of engines and their tractive effort.