Sounds like your requirements would synch with
ACA's Northern Tier very nicely. Amtrak has two lines:
Lake Shore Limited and
Empire Builder that parallel the N.T. allowing you to enter and leave it at various points. Look at the route online and see what 1K mi. section seems to fit your interests the most. Each section has elevations stats, either east or westbound. The beauty of the ACA routes are that they are designed to get you off of the most traveled roads. They give you all the info you need for camping, food, repair, etc.
Have fun, whatever route you choose.