4-8-8-4 Big BoyThe only Union Pacific Big Boy 4- 8-8-4 locomotive to be preserved
east of the Mississippi River, #4012 was at Steamtown's former Vermont
site before moving to Scranton, Pennsylvania to become part of the
Steamtown National Historic Site.
The Big Boy 4000-class was created to move heavy loads over the Wasatch Mountains on the Green River to Ogden line- indeed, the type's intended name was apparently meant to have been Wasatch. Built by Alco, the Big Boy design was an extrapolation of the earlier UP 4-6-6-4 Challenger, with extra driving wheels, a stretched boiler, and a larger firebox. 4012 was one of the first batch of Big Boys, and was built in 1941.
At least at one point, 4012 was considered to be restorable to operation, but it was uncertain wether the infrastructure in the area could handle operations by such a large, heavy locomotive. As it is, it cannot fit on the Steamtown turntable, and is relegated to outside display near the roundhouse.