Scenery will be great for you adults, not so much for the little one. Here's some advice from our past trips (mainly longhaul air) with a toddler:
Buy a number of inexpensive toys--small ones are obviously best--and dole them out slowly so that there is a novelty factor. (Dollar stores are great for this!) Likewise, have a large supply of small snacks, perhaps indulging your child with things that ordinarily you would limit at home (like cookies, candy, etc.) The idea is, every waking hour should feature some new and fun things for her. Of course, some of her tried and true toys should make the trip, too.
I would not recommend the Sightseer Lounge, though. She won't want to sit still there, and corralling her against her will may be a problem. I'd use the bedroom as your play area, and the hallway outside the bedroom is good for walks back and forth (and silly races, if she likes that sort of thing). Be sure to carry her, though, when you are transiting between cars--too much of a chance of a fall or hand pinch there. In the bedroom you have a large window for your own viewing pleasure, and if you leave the door open, you can see out the other side too. Not quite as good as the Lounge, but a better compromise. You can call her attention to interesting things (to her) outside the window--"Look, some cows!" "Look, a big truck!"
Bring a small blanket from home that she uses; it will smell 'right' to her and be a comfort. And don't forget her favorite stuffed 'lovey,' for sure! Download some of her favorite children's videos on a tablet or smartphone (and have some over-the-ear headphones for her so the adults don't go mad on the tenth running of Curious George!)
Dining in the dining car is a good adventure, and luckily there is nothing on the table that can't be safely grabbed and handled by a small person. While ordinarily people are seated four to a table, I'm guessing that they may well let the three of you have your own table. Here's what I would do: On your first trip to the dining car, ask the lead dining attendant these two questions: "I know that seating is four to a table, but with a toddler here who doesn't recognize personal boundaries always, I'm afraid the other guest might have a difficult meal. Would it be possible to get a table just for us three? And my next question is, do you guys pool tips here in the dining car?" That couples your request for a variance in policy with a hint that you'll be tipping for good service. There are supposed to be booster seats available, but that's been a bit hit or miss lately. I'd ask your sleeping car attendant to check on that for you before the first meal, so that if they aren't available, you can consider the option to have your meals in your own bedroom instead. That is a good option anyway if mealtime happens to coincide with a time that your toddler is not in the mood to sit quietly at the table. Oh, and speaking of amusing a little one while you wait for food to arrive, Grandma here recommends having a container of multi-colored Trix cereal to play with and to eat while waiting.
If you're in the middle of potty training, this might be a good time and place to postpone it. Train bathrooms are strange and maybe scary...
Regarding sleeping: Naps and ad hoc sleeping will work fine on the couch area of your bedroom, but for evening sleeping, the bottom bunk is generously sized for an adult and a little person. Movement on the train can keep us awake, but most toddlers fall asleep in the car as soon as you drive off if they're tired--I expect the train will be the same.
Actually, now that I think about it, traveling with a toddler isn't so different from everyday life with a toddler, and it's probably easier than traveling with a slightly older child who might be bored or whiny.
Let us know how it goes!
