Union Pacific 3985, also known as the "Challenger", is a four-cylinder simple articulated 4-6-6-4 "Challenger"-type steam locomotive built in July 1943 by the American Locomotive Company (ALCO) of Schenectady, New York, for the Union Pacific Railroad. No. 3985 is one of only two Challengers still in existence and the only one to have operated in excursion service. No. 3985 operated in revenue service until 1957. It was then stored in the roundhouse in Cheyenne, Wyoming, until 1975, when it was placed outdoors beside the Cheyenne depot. In March 1981, after a group of Union Pacific employees restored the locomotive to operating condition, it was placed into excursion service as part of the Union Pacific's heritage fleet and became the world's largest operational steam locomotive. Mechanical problems took it offline in 2010, after which it was stored at the Union Pacific's Steam Shops in Cheyenne. On May 1, 2019, the title of largest operational steam locomotive was passed to the newly restored 4-8-8-4 "Big Boy" Union Pacific 4014. Because of its poor mechanical condition, No. 3985 was officially retired from excursion service in 2020. In April 2022, UP officials announced that the company would donate the locomotive to the Railroading Heritage of Midwest America (RRHMA), which is now restoring the locomotive to operating condition. History Design Designed by UP chief mechanical engineer Otto Jabelmann in 1941, UP 3985 was part of the second order of this second version of the Challenger. The design drew on recent experience with the enormous 4-8-8-4 Big Boy locomotives, and resulted in a locomotive in that in working order weighed some accompanied by a tender that weighed when two-thirds loaded. Calculated tractive effort is . The Challenger class was intended to speed freight operations on the 0.82% grades across Wyoming, while the 1.14% Wasatch Range climb east from Ogden was to be conquered by the Big Boys without helpers. The Challengers and Big Boys arrived on the scene just as traffic was surging in preparation for American participation in World War II. Revenue service and retirement (1943–1957) UP 3985 was part of the 4664-4 group of Challengers built in 1943. Although this group consisted of 31 locomotives, only 25 went to the Union Pacific. UP 3985 operated in its last revenue train service in 1957, and the locomotive was officially retired in 1962. The following year, No. 3985 was repainted and put on display in Cheyenne,