FEF-3
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 insure years of mainline service. These were the last steam locomotives delivered for the UP. 844, the last of the FEF-3 class, is the longest continuously operating 4-8-4 engine in the world, and the only one never retired by a Class I railroad. Like many of the "late era" steam locomotives, their final design was cut short by the advent of new monarchs of the rails, diesels. "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" (Steve A. Lee, Manager of Union Pacific Steam Program).
Read more about this topic: Union Pacific FEF Series