Union Pacific 3985 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.

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.

In May 2019, the title of largest operational steam locomotive passed to the newly restored 4-8-8-4 "Big Boy" Union Pacific 4014.

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 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.

3985 was repainted and put on display in Cheyenne, Wyoming, for the 1963 National Railway Historical Society (NRHS) Convention alongside Big Boy No.

[1] Beginning in 1979, a group of Union Pacific employee-volunteers started work on restoring the locomotive and it was returned to operating condition in March 1981.

[1][5] The locomotive made its first excursion run in May during Railfair 1981, where it attended the opening of the California State Railroad Museum in Sacramento, along with No.

[1][6] That same year, the locomotive pulled a 143-car doublestack train between Cheyenne and North Platte, Nebraska, per special request by American President Lines.

[8] The next year, the 3985 went to the National Railroad Historical Society's convention in San Jose, California.

3985 travelled from Cheyenne to Omaha, Nebraska, where the locomotive was scheduled to pull an excursion to Chicago, Illinois for the 1993 NRHS Convention, and it marked No.

[12] A number of UP's routes in the Midwest took a huge hit during that year's Great Flood, including the Sedalia Subdivision in Missouri, where No.

[13] After the convention ended, the locomotive travelled to Kansas City, where UP experienced a power-shortage; No.

3985 was used to pull honest freight trains around the area prior to its return to Cheyenne in August.

3985 pulled an excursion on California's Cajon Pass with the Union Pacific's A-B-A set of EMD E9 units during its 1994 tour.

[15] After the event, the 3985 was forced to pull the return trip with the 844 dead in tow after the FEF-3 suffered a tube failure on June 24 while on display.

[19][20] In December 2018, Union Pacific asked the Federal Railroad Administration (FRA) for waivers to exempt Nos.

3985, 844 and 4014 from federal Positive Train Control (PTC) requirements;[20] in February 2019, the FRA officials responded that such waivers were not needed.

[27] In November of that same year, UP moved 3985 and the rest of the donated equipment to the RRHMA's large shop facility in Silvis, Illinois.