This unit and one B unit were equipped with 12-cylinder Cooper-Bessemer FVBL-12 diesel prime movers (these two were informally known as Model UM-16, 1600 hp), while the other A-B units were equipped with 8-cylinder C-B FVBL-8 diesels (informally known as UM-12 for the 1200 hp they produced). The purpose was to test existing designs in road locomotives. Alco had upset GE by not bidding on a couple of locomotive contracts that GE was interested in, so GE set out on its own to build a road diesel, which in the long run proved fatal to Alco. From 1954-1959, the UM-20 (also known as UM-20B) tested on the Erie, painted in Erie colors and numbered #750A-B-C-D. Since GE's design department had a hand in locomotive designs at Alco, there is a similarity between this design and the FA-1/FA-2 Alcos. These locomotives lead to the design and development of the U25B. After the Erie road tests, the locomotives were returned to GE, where all four units were upgraded to 2000 horsepower locomotives with 12-cylinder C-B FVB12LT diesel prime movers, and sold to the UP on October 21, 1959 as #'s 620-620B-621B-621. They were retired by the UP in 1963 and traded to GE on U50's.