SNHS Cab Tour: Union Pacific "Big Boy" No. 4012

No. 4012 is among a unique class of 25 articulated steam locomotives with a 4-8-8-4 wheel set built solely for the Union Pacific Railroad by the American Locomotive Works (ALCO). Most were constructed in 1941 (including No. 4012) on the eve of the United State's entry into World War II. Their ability to haul unassisted heavy loads subsequently played a critical role in America’s home front war effort. An engineering marvel that is perhaps the high point of steam locomotion technology, the “Big Boys” nonetheless became obsolete as more efficient, reliable, and cost-effective diesel-electric locomotive technology was refined during the 1950s. The last Union Pacific “Big Boy” revenue run occurred in 1959.