Classified by the UP as an FEF-3 (for Four Eight Four, third version), No. 844 and its FEF-class siblings had their origin in a 1937 incident involving what was then the reigning passenger power on the UP, the 4-8-2 Mountain. As related by author Brian Hollingsworth, One day in 1937, a 7000-class 4-8-2 had the temerity to demonstrate the lack of steaming power inherent in the type, on a train with UP President William Jeffers' business car on the rear. Even while the party was waiting out on the prairies for rescue, a dialogue by telegram went on with Alco in far-off Schenectady, with a view to getting something better.* 

The result was a group of 45 FEF-class 4-8-4s that, like other exemplary Northerns of the period, represented the ultimate expression of the art of steam locomotive design. Delivered in three orders as classes FEF-1, FEF-2, and FEF-3 in 1937, 1939, and 1944 respectively, they regularly cruised at over 100 mph and were known on occasion to reach their design limit of 110 mph. To reduce the reciprocating mass of their main and side rods - always a design issue with high speed steamers - they were fitted with unique tapered rods. The side rod bearings on each of their 80 drivers were also specially designed to reduce strain and allow higher speeds. Like the Niagaras that Alco would design just a few years later for the New York Central, the oil-burning FEF class had a massive cast steel frame, roller bearings on all axles, and a clean, uncluttered boiler atypical for an American locomotive. 

* Brian Hollingsworth, The Illustrated Encyclopedia of the World's Steam Passenger Locomotives, Salamander Books Ltd. 1982