ALCO built the S-2 from August, 1940 - June, 1950, and the S-4 from June, 1949 - August, 1957. Power for the S-2's and S-4's came from ALCO's 1,000hp 539T straight-6 prime mover. The initial CR roster listed only PC-inherited S-2's in seried 9633-9660 and S-4's in series 9695-9701, as well as both units in non-consecutive series in the 9700's and 9800's. The S-2 was delivered with ALCO Blunt trucks, while the S-4 came standard with AAR type A switcher trucks.
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