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!

Yes!

That train is the Bernina Express.
A great ride, both ways.

+1,000!

Fantastic views + train engineering marvels + lunch in Italy. What's not to love?

Take the route that starts from St. Moritz - you get the best views of the Glacier Express without the long journey from Zermatt.

Trip from Machu Picchu back to Cusco after hiking for 4 days. Train has glass ceilings. Beautiful views. Fashion show on board. fun!

I've done the Canadian in sleeper class both ways across from Toronto to Vancouver a few times. It's pretty affordable if you get a berth, and book from the via rail "deals" website. The food is top notch, but it starts to get a bit boring after while on the four day trip. I would recommend just going from Winnipeg or Edmonton to Vancouver or vice versa.

I also really enjoyed the Coast Starlight (SEA-SFO) (was unlucky enough not to get a Pacific Parlour Car), and the Empire Builder (SEA - CHI) is a great trip as well. It's too bad that Amtrak removed the wine tasting though.

See my signature ;)

The Spanish AVE high-speed train that operates direct from Seville to Barcelona, bypassing Madrid. It's a journey of about 650 miles accomplished in about 5 hours, 40 minutes, with a top speed of about 190mph.