How many Big Boy locomotives were built?

Twenty-five Big Boy locomotives were built for the Union Pacific Railroad.

The American Locomotive Company produced the articulated 4-8-8-4 locomotives in two batches: 20 engines in 1941, numbered 4000–4019, and five more in 1944, numbered 4020–4024. They were designed to haul heavy freight over the steep Wasatch Mountain grades between Ogden, Utah, and Green River, Wyoming.

Big Boy’s wheel arrangement used a four-wheel leading truck, two groups of eight driving wheels, and a four-wheel trailing truck. This allowed the enormous locomotive to negotiate curves while supporting its large boiler and firebox. The class was built for freight rather than speed, although its design speed was about 80 mph.

Most Big Boys were scrapped after diesel locomotives replaced them. Eight survive today: seven in static preservation and Union Pacific No. 4014, which returned to operation in 2019 after an extensive restoration. The number 25 refers to the complete production run, not the number still preserved.