Hi Normb6e, Welcome to the Trainz forums,
Here at the TPR Download Depot -
http://trainzproroutes.com/downloads/index.php?act=category&id=73
A fair few RS-11 and RS-18 switchers.
Also, try searching the DLS for 'RS3, RS-3, S-3, S3', that'll probably bring up a few items.
Try flicking through here -
http://trainz.banal.net/catalog/catalog.html
On the very right hand side of the screen is the US section. The locomotives are listed by railroad name. Say, 'Union Pacific EMD' will show all the EMD locomotives for UP that 'obo' could find. To find Union Pacific ALCo units, for example, you would look under 'Union Pacific All Others'. There's heaps of road names, most haven't got 'EMD' or 'GE' after their name, so you'll probably have to search through all of them. When you find a loco you want, have a look at it's little info box thing, and it'll say what it is, what road, and where to find it.
It's a very useful tool, that site. I'd put it in my favorites if I were you, it comes in handy.
Hope this helps,
Mitch
P.S. If you like little switchers, try searching the DLS for '44 tonner'. It's a GE locomotive, but still a boxy switcher.
P.P.S The Union Pacific section was just used as an easy example, there's only an ALCo PA1, PB1, and a Fairbanks-Morse Erie unit. These are mainline units!