The Union Pacific Big Boy has 16 driving wheels. Its 4-8-8-4 wheel arrangement includes two groups of eight coupled driving wheels, plus a four-wheel leading truck and a four-wheel trailing truck.
The Big Boy was an articulated steam locomotive built by the American Locomotive Company for Union Pacific. The design grew from the Challenger locomotive and was intended to move heavy freight over the Wasatch Range, where steep grades had required helper engines and double-heading operations.
The driving wheels were the large powered wheels beneath the locomotive’s two engine units. They should not be confused with every wheel in the 4-8-8-4 arrangement: counting the leading and trailing trucks produces 24 locomotive wheels overall, but only 16 are driving wheels.
Union Pacific ordered 25 Big Boys between 1941 and 1944. Their 68-inch drivers were smaller than those of many fast passenger locomotives, helping provide the high tractive effort needed for heavy freight rather than maximum speed.