The Union Pacific's Overland Route, the eastern portion of the Transcontinental Railroad, was built west from Omaha, across Nebraska and Wyoming, and on into Utah. The steepest grade was the eastbound climb on the Echo Canyon line through the Wahsatch Mountains just east of Ogden, Utah.

Forty 4-6-6-4 Challenger locomotives were acquired in 1936 and 1937 to move fast freight over the grades in Utah and Wyoming. They were rated at 4,290 tons across Wyoming, but were limited to 3,100 tons eastbound through Echo Canyon. Union Pacific wanted something that could make the same speeds
as the Challengers but could carry the entire 4,290-ton train over the Wahsatch Mountains without a helper. The easiest solution was to scale up the successful Challenger design by adding another pair of drivers to each half of the locomotive thus making a 4-8-8-4.

In 1941 UP placed an order for twenty 4-8-8-4's, numbered 4000 through 4019, with the American Locomotive Works. Each engine cost $265,174. According to legend an unidentified machinist at the ALCO plant is responsible for the name "Big Boy", having scrawled the name in chalk on a partially
completed locomotive.

The Big Boys were exactly what the railroad wanted. They were coal burners with 68-inch drivers, 135,375 pounds of tractive effort and 6,000 horsepower. They started service on the line from Ogden to Green River, Wyoming and their operating range soon increased to cover the line all the way to
Cheyenne. Traffic during WWII resulted in 5 more Big Boys, numbered 4020 through 4024, being built in 1944. These versions were slightly heavier than the original order due to wartime materials restrictions.

Despite the influx of diesel locomotives following WWII, the Big Boys and Challengers remained the prime power on the Overland Route. They also saw service as helpers, leading gas turbines and diesels over Sherman Hill. They remained active through the 1950's and weren't retired until the early
1960's. By then the first twenty units had been run well over one million miles. Eight of the twenty-five Big Boys have been preserved in the following locations:
#4004 in Cheyenne
#4005 in Denver
#4006 in St. Louis
#4012 in Scranton
#4014 in Pamona
#4017 in Green Bay
#4018 in Dallas
#4023 in Omaha