They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to the Portland Rose and other passenger trains.

Technical breakthroughs allowed the UP Challengers to operate with 280 lbf/in2 (1.93 MPa) boiler pressure, something usually reserved for passenger locomotives like the FEF Series.

[clarification needed] Speeds in excess of 60 miles per hour (100 km/h), while unheard-of on most other railroads using articulated steam locomotives, became commonplace on the Union Pacific.

When the first Challengers entered service in 1936, on the UP's main line over the Wasatch Range between Green River and Ogden, the locomotives had problems climbing the steep grades.

Using the experience from the Big Boy, UP chief mechanical engineer, Otto Jabelmann, redesigned the last three orders of Challengers in 1941.

From 1941, the Challengers were intended to speed up freight operations on the grades across Wyoming; the Wasatch Range climb east from Ogden was taken over by unassisted Big Boys.

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

3985 was restored to operating condition by Union Pacific in 1981 and used in excursion service until mechanical problems sent it back into storage in October 2010.