The UP 4-8-2s were classy machines that were very powerful. But they had a major drawback, they were mediocre steamers. It was not uncommon for a MT to run short of steam and have to stop or slow down while running a fast train. Union Pacific put up with the problem until one of the engines had to stop on the main with a fast train which was pulling the RR president's private car. Even before the rescue engine arrived to help the train Union Pacific had contacted ALCo with a view of something with better steam raising ability. The FEF-1 was the result.
-Ben
-Ben