Get a bunch of little toys--dollar stores are a great place to check for this--and dole them out for times when she gets bored or fussy. (We did this for our daughter on long haul flights to the UK when she was little.) Crayons and paper, little tubs of Play-do, mini stuffed animals, etc. Make sure you bring her 'special' lovey--whether it's a blanket or stuffed animal, and if she uses a small blanket for naps, bring that too--and don't bother washing it, since it will have her comforting smell on it! Nowadays, there are online videos that she might enjoy, though be sure to download what you need from home, since there is no Wifi on the trains and cell coverage can be spotty. Snacks work almost as well as toys to amuse and delight.
While meals in the diner can be fun, they also can be a challenge for very little kids to have to wait for food. You might tote some little neat snacks for her to nibble on while waiting for food. And don't discount the play possibilities of stacking little creamers or jelly tubs...
And don't forget to bring plenty of baby wipes for tidying up little hands and faces--yours too! Paper towels are good to deal with messes, and some Clorox wipes are useful for wiping down surfaces in the roomettes--I'm afraid things are not as scrupulously clean on the train as you are likely used to at home! Plastic bags are helpful for containing messes, especially anything stinky, and if well tied off, will be appreciated when discarding said messes in the trash containers in the hallway.
Story time on the train is another--favorite books will add another familiar aspect to the unfamiliar train setting. Thomas the Tank Engine might be fun! Spotting things out the window is both fun and educational, "Hey, look! See the cows? Those cows look hungry, they are eating the grass." And a walk to the lounge and through the coaches is both exercise and something new to do.