If you have an idea of the prototype (which railroad you want to do), then guessing which locos were in use when becomes moot.
True. Of course, unless you are adhering absolutely to the prototypes' motive power, there can be room for some leeway. For example, years ago I built a kit of a Pennsy 0-6-0 switch engine. The Pennsylvania Railroad favored Belpaire fireboxes on its steam locomotives, which most railroads did not, going with radially-stayed fireboxes (the latter being round like the rest of the boiler, whereas the Belpair firebox has squared-off bulges that stick out at the top of the boiler, in front of the cab). Now the Chicago, Burlington & Quincy R.R. pretty much used radially-stayed fireboxes on its steamers. But its parent, the Great Northern liked the Belpaire design, and many of its steamers reflected that policy. Since I model a ficticious subsidiary or division of the "Q", it was easy enough to decal the locomotive to either G.N., or my G.G. & W. and simply say the loco was on loan/lease.
In actual fact, the "Q", GN and Northern Pacific and the "Q"'s subsidiaries, the Colorado & Southern and Fort Worth & Denver leased and loaned (pooled) equipment to each other, long before the I.C.C. permitted them to merge the books into the BN. (In fact, the GN and NP jointly owned the C.B. & Q., and had interlocking directorships dating back to the late 19th Century!) Before the "merger" it wasn't unusual to see GN's orange and green striped Empire Builder pulled between Chicago and Minneapolis by silver Burlington E-unit (diesels). And the NP's North Coast Limited got a similar treatment. Some of the passenger cars decorated for the name trains might have the C.B.&Q letters in the upper corner of the other road's cars!
Can't speak about Eastern railroads, but I suspect there was some of this that went on. If California, Oregon, Washington and even Nevada are what you want to model, even freelanced, you've got the Southern Pacific, West Pacific, and even some of the logging railroads in the forested mountains could be a possibility. I'd love to have an SP cab-forward articulated on my pike, but that's stretching things, and I'm not sure how well a 2-8-8-4 would negotiate 20" radius curves. OTOH, an SP GS-4 in Daylight paint scheme might work, if it has the center two sets of drivers blind (no flanges on the drivers).
Anyhow, the most important thing is to have fun!
Green boards all the way!
Trailrider