The GE Universal Series locomotives were diesel locomotives originally launched by GE in early 1956. The latest variant in the Universal Series was built in 2005. Initially, there were nine U-series models: U4B, U6B, U9B, U9C, U12B, U12C, U18B, U18C and UD18B. They were designed to be readily adaptable to different track gauges for export markets and very reliable. The U5B and U8B appeared later, being first built for RFFSA, Brazil in 1961.
The Universal Series has at its heart the Cooper Bessemer engine. General Electric built four demonstrators in 1954 with Cooper Bessemer engines for road testing. This demonstrator set was numbered the 750ABCD. The demonstrators were made up of two cab units and two booster units. The demonstrators spent considerable time testing on the Erie Railroad. The 750AB were UM12s built with eight cylinder, FVBL-8 engines rated at 1200 horsepower. The 750DC were UM18s built with 12 cylinder, FVBL-12 engines rated at 1800 horsepower. After testing the units were rebuilt in October 1959 at GE-Erie with 16 cylinder engines rated at 2000 horsepower. The former demonstrators were sold to the Union Pacific as UM20s in April 1960 and numbered UP 620AB-621AB. The UM20s were retired in October 1963.
While the GE 750 demonstrator was still testing GE built two XP24 demonstrators, numbers 751 and 752 in 1959. These units became the first U25Bs. The GE U25B became the first widely sold domestic market GE Universal Series locomotive. North American Universal Series locomotives are often called "U-Boats" by railfans and model railroaders.