There is a town of Dillon (pop. 4200) on the UP, making a connection at Butte, so that is a candidate name for the optional new 3rd town.
Or, if I want to follow the prototype more closely (not sure that I do), I could center the layout at the yard at Helena, with the BNSF branch to Great Falls, short branches to Deer Lodge at the left (west) and Three Forks at the right (east), and a connection to the UP at Silver Bow (optional 3rd town instead of the restroom). This makes for a nice/suggestive set of place-names: Cascade, Wolf Creek, Deer Lodge, Silver Bow, Three Forks, and then Helena Yard. [Cascade, Wolf Creek and Deer Lodge are actual towns with sidings on the rail line, that I added in the correct location as they were left off of the MRL map from the Internet, as they are actually on the BNSF.]
No Norfolk Southern this far north and west, that is a bit of a fudge/imagination. The branch to Deer Lodge is on the BNSF, along with the branch to Wolf Creek/Cascade.
Back in history, the NP connected from Whitehall/Spire Rock to Butte, and the Milwaukee Road from Sappington to Butte; thus making a big oval like the layout! The NP and Milwaukee from Butte on different routes, met at Deer Lodge and then ran parallel west on separate mains (the NP exists as MRL and BNSF, the Milwaukee is gone).