You are likely to see the locomotives and rolling stock of ANY railroad in this country anywhere else...so there's nothing that says you can't model your area and put some NS equipment in there.
But you just said it is a "free-lance" road...so there's nothing at all wrong with modelling the area you live in but running the equipment of another railroad on it.
But just for the record...the area you live in would be BNSF and UP territory, but I work for the NS here in the Atlanta, GA area, and we have tons of BNSF and UP locomotives on our route all the time. In fact, there is a rumour going around that BNSF is looking at merging or buying the NS, but I don't know if there's any truth to that. I always joke that if they do, they won't have to add anything but a B and a F to the locomotives, because the NS part of the BNSF is already there!