I enjoyed watching this: http://www.bbc.com/news/magazine-29379678 and actually doing it might be pretty good.

I have done Empire Builder Milwaukee-Seattle, Coast Starlight Seattle-Salinas (wrong direction if you want to see the scenery!), and California Zephyr Sacramento-Chicago and it was a great trip. The attitude of Amtrak staff throughout put most airlines to shame. The food wasn't too bad and the Empire Builder seemed to be in a competition with the Starlight with their wine tasting (it was free for one thing). Unfortunately the Empire Builder is very unreliable at present owing to the route being clogged with oil trains.

I really like the TPE between Manchester and Thirsk, but that's because it takes me back to growing up in the area!