Unless you’re set on a theme west of the Mississippi, you might look into some of the eastern railroads’ “branch” lines. I plan on someday modelling the Reading’s Bethlehem branch, as it passes through the town I grew up in (though I was there long after the reading was gone, and the line a mere shadow of its former self). In it’s heyday, it ran from Philadelphia/Jenkintown up to the large Saucon yard near Bethlehem Pa, servicing Bethlehem steel and interchanging w/ the LV RR. 2-track all the way, many towns along the way with noteable facilities at Lansdale (where I grew up) and Quakertown PA. In addition to running the steel plant freight, there was significant local industry on the route, as well as daily passenger service from Philadelphia to Bethlehem, not to mention interchanges with other branches and railroads. In fact, though a “branch” line, you’d be hard pressed to model the entire thing (even with compression) in anything less than a large basement. Lack of traffic would not be an issue.
I’d imagine many of the eastern lines like Reading, PRR, and NYC would have many branches like this.