At left is Athearn's February 1974 advertisement announcing the coming Baldwin S-12 to the company's loco roster. The S-12, built between 1951 and 1956 with 451 examples made, is the only Baldwin prototype to join the Athearn diesel collection to date. Mid 1974 is the date Athearn's Baldwin S-12 arrives on the market. The May 1974 edition of Railroad Modeler magazine includes the Athearn S-12 Baldwin reviewed in the "A Closer Look" section. Athearn's selection of roadnames and roadnumbers appear to pass the prototype check, except for the Santa Fe #2200. This was not a Baldwin S-12 owned by Santa Fe, but a predecessor model DS 4-4-1000 built in 1948.

The Baldwin Locomotive Works built 451 examples of the S-12 diesel switcher between 1951 and 1956. A wide variety of North American railroads adopted the Baldwin S-12. From major roads like Pennsy, B&O, KCS, and SP to small roads and shortlines like McCloud River, Sierra Railroad, Apache Railroad, and Tennessee Valley Authority. The S-12 also dressed for military service by the United States Air Force, Army and Navy.