Favorites:

• Rhine Valley in Germany - Very scenic route along the river where every hill seems to feature a vineyard or castle (or both).
• Bangkok to Chiangmai - Fun sleeper car trip, especially with friends. It's about 13 hours and the sights include temples and monkeys.
• Eastern Switzerland - I can't imagine any train ride in Switzerland isn't severely scenic, but I really liked the pace of the tiny train lines once you get far out.