To replace inefficient steam switchers, ALCO introduced the S series as a successor to the HH series in 1940. The new engine in these locomotives was now marketed under the ALCO brand, while the earlier switchers had engines named after the name of ALCO's subsidiary McIntosh & Seymour. All models had two trucks with two axles each, electric power transmission and a cab at one end. In comparison with the HH series the hoods were now lower, providing a better view from the cab. A considerable number operated well into the 21st century.

The S-1 had a naturally aspirated engine delivering 660 hp. In 1950, it was replaced by the S-3 that had the same power, but new trucks. In total, 543 S-1 and 300 S-3 had been built by November 1953. As a more powerful variant, the S-2 was also introduced in 1940 with a turbocharged engine delivering 1,000 hp. As early as in 1949, this one was replaced by the S-4 with new trucks. By 1957, production had totaled 1,502 S-2 and 797 S-4.

In 1954, ALCO started their first try to introduce their new 251 engine into the switchers. So they offered the S-5 with this engine, but since it only delivered 800 hp, only seven were built. But the S-6 with a 900 hp variant of this engine that was offered starting in May 1955 was a success. By December 1960, 126 had been built. The Oliver Iron Mining Company received two cow-calf pairs designated SB-8/SSB-9. These consisted of one regular S-6 and a second one without a cab, which were permanently coupled together.