Good travel games for two players, especially for playing on trains?
I would suggest Cribbage. Not much space is required, it is quick to set up and put away, involves no more noise than any game between two players, and is a very good game. It is also best enjoyed with a glass of something in the other hand, increasing its suitability for a train journey.

If you don't know how to play (and it isn't terribly complicated, although rather different from other card games), have a look here... http://en.wikipedia.org/wiki/Cribbage