While EMD and ALCO offered 3,000 hp diesel locomotives as early as 1965, GE delivered some U25Cs uprated to 2,750 hp. The first six-axle U28C with 2,800 hp were delivered in November 1965 and the first four-axle U28B in January 1966. In the same year the power was increased to 3,000 hp, which led to the U30B and U30C. The 3,300 hp U33B arrived in December 1966, while the U33C did not follow until 1968.

The first U28s were visually similar to the U25, but changes such as the shortened front hood were only introduced later. The U30B and U30C were able to reach a total of almost 900 units because EMD's production was at capacity. The U33C also made it to 375 units. The Southern Pacific accounted for 212 U33Cs. Of the U30C, 180 went to the Burlington Northern and 150 to the Union Pacific. 

For use with passenger trains, the Santa Fe ordered ten U28CGs, which were externally similar to the U28C. This was possible because there was already a place reserved for the steam generator. The water required for this was made possible at the expense of a smaller fuel capacity. The six U30CGs also went to the Santa Fe. They now had a full-width car body. After derailments, the passenger variants were only used for freight service.