Like the other Union Pacific steam classes, the acronym comes from the first letter of the spelling of its wheel arrangement: Four Eight Four.

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.

The result was the FEF series, a superb class of locomotives that could run at 100 mph and produce between 4,000 and 5,000 drawbar horsepower.

Fitting ALCO's lateral motion devices to the leading coupled wheels eased the negotiation of curves.

[citation needed] Despite frequently running faster than 100 mph (161 km/h), the forces and stresses on the coupling and connecting rods were kept within acceptable limits.

Their improvements included larger cylinders, better tractive effort, smoke deflectors on the sides of the smokebox, and driving wheels that were three inches wider in diameter.

To safeguard operations, UP converted the 800s to burn oil, and fitted a 6,000-US-gallon (23,000 L; 5,000 imp gal) tank in the bunker space.

A former manager of the Union Pacific Steam Program said, "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.