On the other hand, an RSC-2 is basically just an RS-2 on trucks with a center idler axle, and the NYC did have RS-2.
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The only real motivation for buying RSC-2 instead of RS-2 was if you didn't think your track was good enough to cope with higher axle-loadings. So the main U.S. buyers were in the Midwest (lots of branch lines serving country grain elevators) and the South (which in the late 1940s was still a very poor, economically underdeveloped part of the U.S.): Seaboard Air Line's corporate successor Seabord Coast Line still had some light-rail trackage in the 1970s, and traded in (some of?) their RSC-2 on lightweight U18B with very small fuel tanks.
As for six-axle diesels with all axles powered (Alco RSD series, EMD SD series, GE with a "C" somewhere in their model designation), remember that the New York Central was the "Water Level Route": low grades, good track, fast freight trains. Until after the Penn Central merger, the people overseeing the New York Central's track thought they could use the highest available locomotive horsepower in a four-axle, four traction motor, unit: largest customer for U25B and U33B, lots of GP40.... New York Central's managers were not dogmatic: they were willing to experiment, and allowed Fairbanks Morse to send a pair of "Train Master" (six-motor units with 2400 hp, which in the 1950s was high) to demonstrate on New York Central lines.