The 25 Big Boy locomotives were built to haul freight over the Wasatch Range between Ogden, Utah, and Green River, Wyoming.

In 1936, Union Pacific introduced the Challenger-type (4-6-6-4) locomotives on its main line over the Wasatch Range between Green River and Ogden.

[3][4] Union Pacific therefore decided to design a new locomotive that could handle the run by itself:[5] faster and more powerful than the compound 2-8-8-0s that UP tried after World War I, able to pull long trains at a sustained speed of 60 miles per hour (100 km/h) once past mountain grades.

[6] The new locomotive was carefully designed not to exceed an axle loading of 67,800 lb (30,800 kg), and achieved the maximum possible starting tractive effort with a factor of adhesion of 4.0.

[12] There was some speculation that the first series of Chesapeake and Ohio 2-6-6-6 H-8 “Allegheny” locomotives, built by the Lima Locomotive Works in 1941, may have weighed as much as 778,200 lb (353,000 kg), exceeding the Big Boys, but subsequent re-weighs of early-production H-8s, under close scrutiny by the builder and the railroad, found them to be about half a ton less than 772,250 lb (350,290 kg).

[15] Unlike a similar effort with the Challengers, the conversion failed due to uneven heating in the Big Boy's large, single-burner firebox.

[18] One final short-term experiment was the fitting of smoke deflectors on locomotive 4019, similar to those found on the railroad's FEF Series, as well as some of their Challengers.

[4][9] Along with the Challengers, the Big Boys arrived on the scene just as traffic was surging in preparation for American participation in World War II.

The Big Boy locomotives had large grates to burn the low-quality bituminous coal supplied by Union Pacific-owned mines in Wyoming.

Water to the boiler was furnished by a Nathan type 4000C Automatic Restarting injector rated for 12,500 gallons per hour on the right side and an Elesco T.P.

Upon their arrival on Union Pacific property in 1941, the Big Boys were assigned to the Utah Division's First Sub, between Ogden and Green River, which included the 1.14% grade for which they were designed.

From February 1943 to November of the same year, three Big Boys were assigned to the Los Angeles & Salt Lake Line and ran between Ogden and Milford, Utah.

In 1944, with the arrival of additional Challengers and the second order of Big Boys, their operating territory was expanded east from Green River to Cheyenne over the Wyoming Division's Fifth, Sixth, and Seventh Subs.

In the final years of steam on the UP, where the locomotives were only fired up to help with the fall rush traffic, the Big Boys saw service only between Cheyenne and Laramie.

Postwar increases in the price of coal and labor, along with the advent of efficient, cost-effective diesel-electric power, spelled the end of their operational lives.