FEF-1: 814 preserved, remainder scrapped, FEF-2: 833 preserved, remainder scrapped, FEF-3:844 in excursion service, 838 in storage as source of spare parts, remainder scrapped
The 45 locomotives were the last steam locomotives built for the Union Pacific. They 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 in 1946. They pulled a variety of passenger trains, such as the Overland Limited, Los Angeles Limited, Portland Rose and Challenger, until diesel-electric locomotives took over passenger service. Many FEF Series locomotives were later reassigned to freight service during the last few years of their careers.
Four FEF Series locomotives survive today, including No. 844, which remains in operational condition and now runs in excursion service. Today, the 844 is one of the Union Pacific's oldest serving locomotives and the only steam locomotive never retired by a North AmericanClass I railroad.
During the late 1930s, the rising trainloads started to exceed the limits of the 4-8-2 that were the mainstay of the UP passenger operations. One day, in 1937, with UP President William Jeffer's business car in the rear, a "7000" Class 4-8-2 demonstrated the lack of steaming power inherent in the type. Even when the train was waiting for rescue, a telegram was sent to ALCO in Schenectady seeking something better. The result was a superb class of 45 locomotives that could run at 120 mph and produce between 4,000 and 5,000 drawbar horsepower.
The first twenty locomotives, numbered 800–819, were delivered by ALCO in 1937. The "800"s as a whole followed – like Northumbrian 108 years earlier – the simplest possible arrangement of only having two outside cylinders. Fitting ALCO's lateral motion devices to the leading coupled wheels eased the negotiation of curves. Complicated accessories often spoiled the basic simplicity of so many US locomotives, but UP resisted most of them, resulting in an elegant, uncluttered appearance. Despite frequently moving at speeds over 100 mph (161 km/h), the forces and stresses on the coupling and connecting rods were kept within acceptable limits. There were thus excellent results, and there were many reports of the class reaching the design limit of 110 mph (177 km/h).
The second batch of fifteen was delivered in 1939. These had several improvements, including larger cylinders, better tractive effort, taller driving wheels, and smoke deflectors on the sides of the smokebox. The greatest change, however, was the provision of a fourteen wheeled “pedestal” or “centipede” tender, in place of the twelve wheeled ones of the first twenty locomotives. Thus, the first locomotives became known as "FEF-1," while these were known as "FEF-2."
Except for the use of some substitute materials, the final batch of ten were nearly identical to the FEF-2. After World War II, coal supplies were affected by a series of strikes. In order to safeguard operations, UP converted the 800s to oil burning, and a 6,000 US gallons (23,000 l; 5,000 imp gal) tank was fitted in the bunker space. Otherwise, few modifications were needed to ensure years of mainline service. These were the last steam locomotives delivered for the UP, and they provided excellent results for crews. On one occasion, one of the engines of the FEF-3 class pulled a 1,000-ton passenger train at 100 mph. Like many of the "late era" steam locomotives, their final design was cut short by the advent of diesel locomotives, the new monarchs of the rails. A former manager of the Union Pacific Steam Program once commented on the FEF series, saying that "although it is stated that the UP FEF Series were designed to safely operate at 120 mph (190 km/h), no one really knows how fast the final 4-8-4 could go."