In 1953, General Electric dissolved its locomotive-building partnership with the American Locomotive Company. A year later, GE produced an A-B-B-A set of cab units under the guise of export units, which were really testbeds for the North American market. The original set consisted of an A-B set with 8-cylinder engines outputting 1200 HP each (UM12B) and a second A-B set with 12-cylinder engines at 1800 HP each (UM18B). They first tested on the Erie as 750A-D, receiving various modifications over time before finally being upgraded to 16-cylinder engines at 2000 HP each prior to sale to Union Pacific in 1959 as 620, 620B, 621, and 621B. UP ran the units until 1963, when they were traded in for U50s. The information that GE learned during tests allowed them to produce a very solid product when they introduced the U25B, and strongly influenced the FDL prime mover that they continued to use for decades.