What is the wheel arrangement of the Union Pacific Big Boy?

The Union Pacific Big Boy has a 4-8-8-4 wheel arrangement.

In Whyte notation, the numbers describe four leading wheels, eight driving wheels, another eight driving wheels, and four trailing wheels. The two groups of driving wheels identify the Big Boy as an articulated locomotive: its frames could move relative to one another to negotiate curves while carrying an exceptionally large boiler and firebox.

The American Locomotive Company built 25 Big Boys for Union Pacific between 1941 and 1944. They were designed primarily to haul heavy freight over the Wasatch Range between Ogden, Utah, and Green River, Wyoming. Later, many worked over Sherman Hill between Cheyenne and Laramie. Their enormous size reduced the need for helper locomotives on steep grades.

The 4-8-8-4 arrangement is distinctive and was used only for the Union Pacific Big Boy class. It is easy to confuse with 4-6-6-4, the Challenger arrangement, which has six driving wheels in each articulated group. Eight Big Boys survive; No. 4014 was restored to operating condition in 2019.