Big Boy was the name of the Union Pacific Railroad's 4000-class 4-8-8-4 articulated steam locomotives, built between 1941 and 1944 by American Locomotive Company. 25 Big Boys were built in two groups. The first group, called "class 1", were built starting in 1941. The second group, "class 2", were built in 1944. The Big Boys were the only locomotives to have the 4-8-8-4 wheel arrangement, which combined two sets of eight driving wheels with a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox.

The designers determined that to pull a 3600 ton train, a tractive effort of 135,000 lbs would be needed. The designers agreed upon the 4-8-8-4 design. Next, the horsepower and cylinder sizes were computed based on 300 psi boiler pressure. Although they weren't planning to pull these freight trains at 80 MPH, the DoRMS designed them for 80 MPH in order to have a sufficient factor of safety built into the design. What resulted is considered by many to be the most successful articulated steam locomotive ever built.