In 1940, Rock Island approached ALCO and ordered a new type of diesel locomotive that would be equally suitable for switching and road service. Instead of the cab units that were widespread at the time, a locomotive with a load-bearing frame and narrower hoods was developed, also known as the “hood unit”. The resulting design was characterized by the fact that there was an unobstructed field of vision when running in both directions. This meant that it could be used without restrictions for switching, but also in road service without any cutbacks. It was usually running with the long hood forward, even if this meant a poorer view forward. Thus the new breed of road switchers was created, which was soon adopted by all North American manufacturers.

A total of 466 of the RS-1 were completed at ALCO and three more at the Montreal Locomotive Works by 1960. This is the longest production run of any diesel locomotive for the North American market to date, and also meant that they continued in production for several years after their successors were discontinued. The first locomotives, which were actually intended for Rock Island, were delivered to the US Army due to the USA entering the war.

Although a steam generator was available as an option, the RS-1 was rarely used in front of passenger trains. In freight service, with its 1,000 hp, it was primarily used on branch lines. The large six-cylinder ALCO 539T proved to be very durable, with many locomotives still being used in the 21st century. The RSD-1 and RSC-1 were developed as six-axle developments for lighter rails, with the latter only four axles being driven.

The RS-2 was built between 1946 and 1950, with 368 being built at ALCO and nine in Montreal. It differed from the RS-1 mainly in the new 244 series V12 engine with 1,500 hp and a slightly more rounded body. The new prime mover did not prove to be as reliable in use as the old one. 64 were delivered to Mexico and 46 to Brazil. The locomotives for Brazil had a gauge of 1,600 mm.

The RS-3 achieved the highest number, of which 1,272 were built between 1950 and 1956, with 146 coming from Montreal. It was created at a time when Fairbanks-Morse and Baldwin were also bringing their road switchers from 1,500 to 1,600 hp to keep up with competitor EMD. 98 went to Canadian customers and 46 to Brazil. Here, too, a six-axle variant with four powered axles was developed as the RSC-3.

To overcome the reliability problems of the 244 series prime mover, 98 RS-3s were rebuilt between 1972 and 1978 with EMD 567 engines. The engines, including the fan system, came from retired EMD E8. These were named RS-3m to distinguish them. A few of these are still in use in 2022, although some RS-3 that have not been rebuilt have survived into the 21st century.