You might find this website a useful resource: http://trainz.banal.net/catalog/catalog.html
It's a categorized list of locomotives from around the world. If I was looking for something specific, this would be my first stop. So you want US Steam? There's a category for that with info on where to get the locomotive and even an image of it.
I know he tries to keep it up to date, (for example the 750 mentioned was only added to the DLS a bit over a week ago) but he can only add stuff that he knows about. I'm also not sure if he is only listing freeware.