Big Boy is the Union Pacific locomotive class that includes the largest steam locomotives ever built.
Union Pacific ordered the articulated 4-8-8-4 locomotives during the Second World War to haul heavy freight over the Wasatch Mountains between Ogden, Utah, and Green River, Wyoming. The engines were designed to move very long trains without requiring an additional helper locomotive on steep grades.
Twenty Big Boys were constructed by the American Locomotive Company between 1941 and 1944. Each was more than 130 feet long and weighed over 1.2 million pounds in working order. Their immense size made them symbols of American steam railroading.
Big Boy is sometimes confused with the Union Pacific Challenger, another articulated steam class. Challengers were powerful 4-6-6-4 locomotives, but Big Boys were larger and used a different wheel arrangement. Several Big Boys survive in museums, and some have returned to operation.