Spring and fall are usually the best travel times weather-wise in the U.S, and you might have a better chance of getting cheap travel tours. Spring comes late to the Rocky Mountains and fall can come early to northern regions.
If you’re going to national parks, you’ll be glad to have advance reservations for accommodations, at least for the first few nights.
Rail passes make train travel worthwhile, but only if you’re not in a hurry. Amtrak sometimes has unpredictable delays, so allow extra hours if you’ll need to catch a flight at your end point.