Explore the train: swing from the curtains, play daredevil standing on the seats as the train rocks and your mother repeatedly pulls you down, see how many times you can bonk your nose with the tray table, run up and down the aisles exclaiming "THIS" and pointing at each new person you encounter.
Enjoy the scenery. When it starts to get dark and you can't see outside, shout "More, More" to see if that resolves the situation. Play, dress up, snack, color, read, play some more.
Make friends with your neighbors. Share snacks and books with them. Alternately you could choose to not make friends with your neighbors and follow Mama's example by yelling at the crazy/drunk lady who felt it appropriate to bellow parenting advice/other random craziness to her fellow passengers. Enjoy dinner in the dining car. Get to know your dinner companions by shouting "This?" every two minutes, prompting your mother to repeat their names (Jean and Charmaine). Laugh about the fact that your meal ($25: 1 entree, 1 milk, 1 glass red wine, 1 slice of "treat") cost almost the same as your fare ($31).
For your last resort, you can turn to everyone else's favorite way to pass time on the train: