When William Jeffer, President of the Union Pacific, got stalled on a grade with his train, the initial order for the FEF series came about. The 4-8-2 class 7000 locomotive used at the time was to be given a more powerful successor for mixed service, which led to the 4-8-4 wheel arrangement. This wheel arrangement was already in use on other US railroads and led to the designation “Four-Eight-Four”.

The first 20 class FEF-1 locomotives were delivered by ALCO in 1938 and had 77 inch diameter drivers. Rolling resistance was reduced with roller bearings on all axles and tapered connecting rods ensured that the moving masses were reduced. This allowed them to travel at speeds of 110 mph with ease and without compromising safety.

The following year, 15 FEF-2 were delivered, now with 80 inch drivers, larger cylinders and a higher boiler pressure. They were now cleared for 120 mph. Another ten FEF-3 followed in 1944, since no diesel locomotives were available for passenger trains due to the war.

The FEF series locomotives are known to produce between 4,000 and 5,000 hp at the drawbar and often traveled long distances in excess of 100 mph. In 1946 all were converted to oil firing, with the tenders now holding 6,550 gallons of oil instead of 25 short tons of coal.

In their last years, the locomotives were mainly used for freight trains before being retired at the end of the 1950s. Number 844 was the only US steam locomotive that was never officially retired and has since been used for excursion trips without major interruptions. In the meantime, it carried the number 8444 and is the only one of four surviving machines that is roadworthy.