The list is too long to reproduce here, but see the page <http://www.sonic.net/~jayreed/preserved.html>.
Back in the day, small industrial locomotives were made in sizes form a couple of tons, up through 44 tons, by a number of manufacturers: Plymouth, Whitcomb, Davenport, GE, Baldwin.. The railroads would charge for in-plant switching, so any outfir which had a lot of inplant switching to do would maintain their own locomotive for in-plant moves. And a number of the class 1 railroads maintained small units, too. For years, on Blakely Island, in Mobile Bay, the Frisco operated a 44 tonner to switch plants on the Island, and many railroads operated smaller units in car shops and in locomotive facilities. there were some larger units. As I recall, the Columbus and Greenville (MS) operated Whitcomb 65 ton locomotives.
But larger units were not as common as the smaller (44 tons and below) units. The reason was labor. According to labor agreements on Class I railroads, and according to some State laws or regulations, a locomotive smaller than 45 tons could be operated by a one man crew (the locomotive engineer); a unit 45 tons or larger required a two man crew, a locomotive engineer and a fireman. So, to answer the question: "what was smaller than a Balwin S", the answer would be a Baldwin 4, 10, 12, 15, 20, 30 and 44 ton switching locomotive, and locomotives in similar sizes from Plymouth, Whitcomb, Davenport, GE, and other producers.
If I remember correctly, TPR has an EMD 15 tonner.
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