According to Kirkland, the horsepower ratings were somewhat adjustable to be competative with the other builders. At the time, EMD and BLW were offering 100 ton units with either 600 or 660 HP, and 120 ton units rated at either 900 or 1000 HP.
The 531 was rated at 600 HP, or 900 with a turbo. M&StL D539, with a somewhat prophetic number, and Westinghouse electrical gear, was rated at 1000 HP, the only 1000 HP 531T.
The non-turbo 538s were good for 600 HP at 700 RPM, or 660 HP at 740 RPM. The turbocharged 538 was normally a 900 HP engine, but could be set up for a thousand.
This flexability allowed Alco's sales department to exactly match the competitor's horespower ratings when quoting units; no "apples and oranges" comparisons. Again per Kirkland, the transmissions were overbuilt and could handle the incremental power increases.
Remember, this was early in the game; there was not as much standardization as later. Railroad CMOs were still used to specifying just what appliances they wanted on their steam locomotives; Alco power reverse on a Baldwin comes to mind here. (And I find it hard to picture a Westinghouse main gen. and TMs on an Alco switcher----)
The 100 ton (nominal) S-1, with it's 660 HP 539 engine and GT552 main generator, and the 120 ton S-2 with the 1000 HP 539T/GT553 pairing, were really the first mass produced "standard" Alco switchers.
Going back to the rating game, years later, the marketing guys at La Grange tried something that no one really fell for. GE had the U-25B, a 2500 HP road switcher. The EMD competitor was at first going to be called the GP-22, but was marketed as the GP-30. 30 is a greater number than 25, but the EMD only produced 2250 HP. That was really an apples and oranges situation, in more ways than one.
There was a possible overlap, depending on your point of view, between the last 531s and the first 538s. Again per Kirkland, the last 531 was Alco shop switcher 4 built from leftover parts and completed September 28, 1939. The last 531 SOLD was Portland Terminal 1004, shipped June 17, 1938. The first 538 switcher shipped was B&M 1101, delivered August 22, 1938. Alco 4, never intended for sale, causes the overlap.
Hope this helps explain what was going on back in the late thirites. Comparing old rosters, and comparing Kirkland against Steinbrenner, can get quite interesting.
