Union Pacific 4-6-4

Prior to the construction of the FEF-1 Class, the Union Pacific had been contemplating building a 4-6-4 for high speed express passenger service rather than a 4-8-4. Fetters had been influenced and inspired by the success of the Hudsons on the New York Central Railroad, and thus was the motivation for proposing a Hudson for the UP.
Very little is known about the development for these engines. Rough, preliminary blueprints were drawn up for the engine, but that was about as far as they got with designing it. What little is known is the engines would've had 80 inch drivers, and they would've been the largest Hudson-types built in the United States.
Ultimately, plans for the proposed Hudson were rejected, and the railroad decided to order a 4-8-4 from ALCO instead. What resulted was the FEF-1, and the FEF-1 did well enough to encourage the UP to go on to own 45 4-8-4s encompassing three different classes.