I was just wondering what engine used in the S1?
Was the same model used in all of them or did they use different models in different years or for different railroads.
Just was wondering. Thanks for your help.

The S1/3 used the Alco (McIntosh & Seymour) 539 in the "660 HP Switch Unit" - Alco didn't start calling them S1/2/3/4 or RS1 until years after Railfans did. The S2/4 used the turbocharged 539T, as did the RS1.

CD

With respect to Alco's use of the S1, S3, etc. model numbers....I believe if you'll check the 1950 loco cyclopedia (pretty sure that's the year of issue) you'll find Alco already using those model number by that time.

I have an original 1952 Alco maintenance manual on them, they're referred to as:
660-HP SWITCHER
1000-HP SWITCHER
1000-HP ROAD SWITCHER
They aren't even referred to with DL-Specification numbers, which I found odd.
I don't doubt the S/RS names were common and accepted, just that they weren't Alco's.
I've also seen how the "AAR Type B" trucks were never called as such, and have Alco publications calling them that...

CD

The question of when did Alco begin using the model numbers that we railfans use (as opposed to their "specification numbers" is partly answered by the 1950-52 Locomotive Cyclopedia. This volume is copyright 1950. In the sections describing the various Alco models the PA/PB is not identified as such, but the FA-2 (as used by Alco, with the dash) is used--but only for the A unit. In a color ad insert by Alco none of the model numbers appear, but in the more detailed pages following the RS-3, RS-C3 (as shown, not a typo), and RSD-4 are so denoted. But none of the S-models or the RS-1 are so described. So at this point in time it's a mixed bag.

But can it be said that railfans influenced Alco's choice of how to describe their loco models as early as 1950? For those noted above, I don't think so.

Now the question is "When did the use of S-1, etc. by Alco begin?