A bit north of the areas khalling mentioned is the town of Rollinsville. It lies at the mouth of a wide (in rocky mountain terms) valley and straddles the Union Pacific rail line on its way to the Moffat tunnel (the model for the Taggert tunnel) , a 6.2 mile tunnel under the continental divide. A dirt road parallels the track and follows the old railroad bed over the continental divide coming out in Winter Park, Colorado. Then on to Fraser which regularly recorded the lowest temperatures in the continental U.S.
There is also the town of Minturn (near Vail) at the foot of Tennessee Pass which is fed by a rail line through Leadville.

You may certainly come visit me in Montana and do some research. I might have some ideas for you but i need to check first.

The Durango to Silverton line goes past a couple of ghost towns (Needleton, Elk Park). They both have a large river flowing by and are nestled among mountains.

Tiny Lysite, Wyoming (pop 200). The railroad ran close to it, it's about 40 miles east of Riverton (pop 9,000). Rails were torn up years ago and part of the roadbed was made into a "rails to trails" recreation route for joggers and bikers.

Literary license is cool. In my novel, Paris, Wyoming, I moved the Wind River mountain range a hundred miles.............. Jim

In Colorado, those places you can get to by rail, you can get to by air, if they are current stops. Here's some places to explore in Colorado: Leadville, Buena Vista, Salida. Pitkin, Creede, Lake City, Ouray, Telluride, Silverton. There are airports around there, but they are easy to shut out due to weather, erosion, etc. alot of planes go down near these areas.

If you want strips of track/road where there is nothing but trees and nature, there is Husted, Colorado. It was in a valley of the Front Range of the Rocky Mountains near Colorado Springs. Fountain Creek, a tributary of the Arkansas, flows past it. Husted's biggest claim to fame was a rather spectacular train wreck that took place in 1909 (you can find articles about the train wreck on the internet) , but it ceased to be a town sometime in the 1950s. The railroad is still used, it is part of a line that runs coal between Colorado Springs and Denver, but all that is left of the town is the concrete foundation of one or two buildings and traces of a dirt road. The land it was on now belongs to the USAF Academy.

On the eastern plains of Colorado near Kansas is Cheyenne Wells, the county seat of Cheyenne County. Served by highways and Union Pacific Railroad. Just south of town is the site of the infamous Sand Creek Massacre where, in November 1864 a small unit of the Colorado Militia attacked a peaceful group of Native Americans camping along Sand Creek. The soldiers killed every one of the 300+ Indians which were mostly old men, women and children. The soldiers cut off the men's scrotums and made them into tobacco sacks, they cut off their penises and made saddle horns, and they cut off women's breasts and made hats out of the skins. The Indians were flying an American flag, a white flag of truce and were under a treaty with the Army, camping where the local Army commander told them to camp.

In the mountains west of Denver is the historic old mining town of Leadville. The Denver & Rio Grande Railroad served the area until it was taken over by the Union Pacific. Baby Doe Tabor and her husband Horace Tabor and their Matchless Mine are fascinating stories. The railroad is still there but the trains no longer run.

The old town of Georgetown is in the mountains and along a very busy I-70. Its trains still run, but only for tourists taking a short ride on the Georgetown Loop Railroad. Look up the history of this old mining town.

This has nothing to do with the topic. Anyone want to come to Chile to work as a housekeeper? Live among a mountain with people who have shrugged. Housing provided.