The Union Pacific Railroad traversed some of the most challenging mountain ranges in the United States. Over these barriers UP hauled large trains of heavy tonnage, often of perishables requiring fast schedules.
Between Ogden and Wasatch the Union Pacific frequently had to rely on expensive helper engines to boost trains up the grades. During the 1930s UP ordered their R&D department to develop an engine that could haul 3600 tons over the 1.14% Wasatch grade without helpers, yet run like a deer along the Plaines. Thus was born UP’s mighty 4-8-8-4 “Wasatch” locomotive. Never heard of the Wasatch? That’s because the official name did not stick. As the first “Wasatch” was being moved out of ALCO, a worker scrawled “Big Boy” upon it. The graffiti became the mantle the engine will forever be known as.
Twenty-five Big Boys were built in two groups, starting in 1941. The first twenty were numbered 4000-4019. Number 4000 arrived at UP’s Omaha facility on 5 September, 1941. World War 2 traffic required a second group of five, numbered 4020-4024, started in 1944. The final revenue freight hauled by a Big Boy was in July of 1959.
I will not explore the debates as to whether Big Boy was biggest, most powerful, fastest, most efficient, etc. Those are topics I read about, not having the expertise or knowledge of. Some great websites for this are: