During World War II Union Pacific operated some of the most modern and powerful steam locomotives ever built. Among them were the famous "Big Boys," the largest steam locomotives in the world, which were unique to Union Pacific. Working with them were the slightly smaller "Challenger" freight engines and the "800-class" high-speed passenger locomotives, as well as hundreds of older class steam engines. Although Union Pacific was among the first of the U.S. railroads to introduce diesel-powered streamlined passenger trains in the 1930s, the capabilities of these powerful steam locomotives made them the mainstay of UP freight operations throughout the war.
The efficiency of diesels eventually overwhelmed steam. Although steam engines were as powerful as diesels, and often faster, their huge appetite for fuel and water and the need for labor-intensive maintenance spelled their doom. Union Pacific quickly began buying diesel locomotives after the war and steam retreated to a stronghold in Wyoming, where the big engines ran their last miles in the late 1950s.