The fifth of 25 Big Boy locomotives built by Alco for the Union Pacific.
The Union Pacific was hauling freight over the Wahsatch mountain range, a grade of 1.14% with the steepest and highest grade being that of Sherman Hill. Even the powerful Challenger 4-6-6-4 locomotives needed helpers over the range, an expensive proposition, costly in both time and manpower. With storm clouds looming over Europe and Asia, the UPRR wanted to be prepared for the extra effort that would be required in the event the United States would participate in the threatened World War.
The Big Boy was designed to haul a 3600 ton train at 80 MPH over the mountain range. (It never ran that fast, the 80 MPH feature was built to give a safety margin, and the trains usually ran at speeds upward of 60 MPH.)
4004 went into service in 1941 and was retired in 1959, logging over 1,000,000 miles in its career. It was donated to the city of Cheyenne in 1963.