We brought travel-size games, cards, magazines, and books. And we used just about everything we had at one point or another. If you think you have brought too many things to do, you haven't — being on a train for three days is a long time!
I was concerned about getting motion sickness on the train since I am prone to that, but it was never a problem. I read for short periods of time just to be safe. Bring a sense of imagination with you too — when we were tired of the games we brought, we started making up guessing games. The best way to truly bond with someone is to be in a train car with them for an extended period of time.