Comment on Engine Number 1 by classictrains
So who is right? What is this one?
This one is either an S1 or S3. If you look at this pic of an S2 here [link] (yes that's me) you can see that the radiator is much longer, and there is one less engine door than on the S1/S3. 
![[link]](https://www.deviantart.com/users/outgoing?http://img191.imageshack.us/img191/4395/cydx224alcos2.jpg){kind=link}
(this one you have here is probably an S3, as it has the later trucks on it)
I can accept that.. I am surprised to find rrpicturearchives.net has an error though.
I get my info from the Field Guide to Trains of North America, by Gerald Foster. Google actually has the page I'm referring to in their Books preview here [link] (click "Preview This Book" at the top left and scroll to Page 6) It's a great book and it's helped me on numerous occasions in the field when I get confused. 