Union Pacific 844 was one of 10 locomotives that were ordered by Union Pacific in 1944 and designated as class FEF-3. The FEF-3 class represented the epitome of dual-service steam locomotive development; funds and research were being concentrated into the development of diesel-electric locomotives. Designed to burn coal, they were converted to run on fuel oil. Like the earlier FEF-2 class, FEF-3 locomotives were designed as passenger engines. They pulled such trains as the Overland Limited, Los Angeles Limited, Portland Rose and Challenger.
Some enthusiasts think UP 844 could pull 26 passenger cars at more than 100 mph (161 km/h) on straight and level track; they say it was designed to safely operate up to 120 mph (193 km/h) with its one-piece cast-steel bed frame, 80-inch (2,032 mm) drivers, massive boiler/firebox and lightweight reciprocating parts, which were common on late-era steam locomotives. (UP's timetables showed a speed limit for steam locomotives of 80 mph.)
From 1957 to 1959, UP 844 was reassigned to freight service in Nebraska when diesel-electric locomotives took over passenger service.
Saved from scrapping in 1960, UP 844 was chosen for restoration and is now used on company and public excursion trains, along with revenue freight during ferry moves.
Over the weekend of October 14, 1990, the UP 844 led a procession of special trains from Kansas City Union Station to Abilene, Kansas, for WWII veterans to celebrate the 100th birthday of President Dwight D. Eisenhower. The “Eisenhower Centennial Special” was a collaboration between the Union Pacific, Burlington Northern and Santa Fe Railroad business fleets, with additional passenger cars provided by the Norfolk Southern and Chicago North Western railroads. Present also in Abilene was General Eisenhower’s command train, code-named "Bayonet", including the British A4 steam locomotive #60008, communication and staff cars from WWII European Theater Operations. The UP 844 consist for that weekend included 21 units, including the tool and power car, a super dome and regular dome, lounge and observation, concession/baggage cars and chair cars in the following order: Columbia River, Western Lodge, Sunshine Special, Sun Valley, Texas Eagle, Portland Rose, City of Los Angeles, Challenger, Columbine, Western Special, Powder River, St. Louis, Iowa River, Harriman, Cheyenne, Cedar River, Chicago, Minnesota and Fox River.
On May 18, and May 19, 2007, the 844 made another rare appearance with Southern Pacific 4449 in the Pacific Northwest for the "Puget Sound Excursion", on the Burlington Northern Santa Fe railroad tracks from Tacoma to Everett, round-trip. This excursion was a tandem run; the locomotives pulled a Union Pacific passenger train consisting of three to four dome cars and several coaches.
Today, UP 844 runs occasional excursion and fan trips along UP trackage every year. On June 25 and 26, 2010, the 844 made an excursion trip to the Milliken, Colorado, Centennial Celebration. On rare occasions, it has been, and still can be used in revenue service.
In fall 2010, the Union Pacific allowed railfans to vote where the coming season's excursion trains will operate.
In September 2012, the 844 participated in UP 150, a celebration of Union Pacific's 150th anniversary celebration, hosted by the California State Railroad Museum in Old Sacramento, California. During the event, 844's tender derailed on tightly curved track from the mainline to the museum. Later in 2012, Union Pacific officials announced that they are trying to acquire the 4-8-8-4 Big Boy 4014 for restoration and operation.
In 2013, 844's gyrating mars light, first installed in 1946, was removed due to the condition of the bolts holding it in place
- Union Pacific 844 is also Southern Pacific 4449's best friend.