What the ride will be like will depend very much on the comfort inside the train, the age of the train, the speed of the train, whether it's pressurized or not, the number of stops and distance between them. It's not unpleasant :)
Take lots of things to do - I see people with portable DVD players and headphones a lot, maybe take your laptop if you want to work or play, perhaps a book, couple of magazines (reading is easy but it's not so easy to write because of the vibrations), knitting, that kind of thing. Basically, anything you can do on an airplane you can do on a train.
Make sure you have all your valuables (money, ID, ticket, other papers) in a small bag that youi can have on you all the time for when you go to the toilet because they tend to be small and cramped (like on airplanes). Don't forget to get up and walk up and down the corridors to stretch your legs regularly.
at 3:57 AM on Mar. 8, 2013