With the advent of diesel traction in various areas of application in the USA, more and more diesel-powered shunting locomotives were procured. In the mid-range, the problem lay in a law requiring a second person in the cab to play the role of fireman for locomotives of all types of propulsion weighing 90,000 pounds and over. As a result, several manufacturers developed diesel shunters just below this limit, of which the 44-tonner from GE was the most well-known model.

In order to ensure an optimal view in both directions of travel, the driver's cab was placed exactly in the middle and raised. There was a hood on each side of the driver's cab, under which was a diesel engine with a generator. Most of the locomotives were powered by two Caterpillar D17000s with eight V-shaped cylinders and 180 hp each. With a maximum of ten pieces each, different six-cylinders were used. This was either the Hercules DFXD also with 180 hp, the Buda 6DH1742 with 200 hp or the Caterpillar D342 with 190 hp. 

Preserved locomotive in February 2009 at the Cowan Railroad Museum, Tennessee

Casey Fleser

Of a total of 386 built vehicles, 276 went to US companies. 47 pieces were created from a special military variant that were delivered to the US armed forces. These had been given a lower cab for use in Europe and weighed down to 90,000 pounds. Of these, 31 were later sold to the Indian Railways and twelve to the Portuguese CP. The remaining four went to Australia, where they first came to the New South Wales State Railways and were listed as Class 79s. They changed hands several times and two of them ended up with the British Phosphate Commission where they were fitted with more modern Caterpillar D3306 engines in 1979.

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.

In 1969 GE succeeded in bringing the 7FDL with 16 cylinders to an output of 3,600 hp. The first model was the U36B with only four axles, which was intended for fast freight trains. The Seaboard Coast Line bought 108 of the 125 U36Bs. With a total of 238 units, the U36C, which was suitable for heavier freight trains with six axles, was more successful. Most of the locomotives went to the ATSF and the Ferrocarriles Nacionales de México. The U34CH was a six-axle passenger locomotive with electric train heating. Since the engine had to run constantly at 900 instead of a maximum of 1,050 rpm when the power supply was switched on, the gross power was limited to 3,430 hp. 32 locomotives were built for the New Jersey Department of Transportation and another one was rebuilt from a U30C for the New York MTA.