Hi Betty, Lots of good info already.
I traveled on a train a lot with my mother when she was using a walker, the cheap hospital kind, not a rollator.
Ditch the suitcase, won't work without a whole lot of practice in addition to some customizing. Start looking for devices that attach to the walker and also get a backpack that you can wear. Maybe buy one of those jackets or vests that is all pockets or sew pockets all over a T-shirt. If you need a walker, then you need to use it right, with both hands. So downsize what you need to carry to fit whatever storage you obtain. Also, weight matters, it has to balance right on the walker or the whole thing will tip over and take you with it. So don't figure you can pack up like a bag lady's cart with leather and liquids!
As you know by now, there is a certain way you walk with a walker, the way you move is different. Walking with a heavy walker is different than a light walker in the same way. So load it up in the house and practice. Also practice up and down steps and ramps, and include cross-slopes. Ramps with cross-slopes are most "interesting" with a heavy walker and they seem to put them in the worst places.
As an alternative to carrying all of your clothes, consider wearing them. All of them, layered, then take them off on the train.
Get a roomette, for sure, and I second what everyone else says about taking the H-room. Since you are going to be using the walker as a luggage cart, essentially, you won't be able to lift it up a flight of stairs or into the upper bunk. Depending on exactly what model you have, it might not even fit down the hallway to a lower level roomette. The H-room will work for sure, and you can take your walker on excursions around the train to see if there are any other workable options for future trips.
Here are some links, you can probably find others. If you sew, you'll save a ton of $.
Spinlife You want walking aid accessories
Diestco I have a lot of their stuff
Abilities Expo A trade show for people with disabilities. Many vendors so you can try stuff out, also demonstrations, workshops (including travel), and expensive toys to try. You just missed the Chicago show, maybe you can get to the Boston show this year. Now that you have a disability, be sure to get to one, trust me on that!
Easy Access Travel This is a travel agent. I'm putting her here because she gave a good talk at one of the Abilities Expo workshops I went to. She included Amtrak in her talk. She is a leg amputee and uses a scooter most of the time. She is friendly and I'm sure you could email her and get luggage and packing info that would be different than mine. If I ever take a cruise, I'll book with her. She knows about stuff like services that will pick up your luggage at home and deliver it to your cruise ship, and I don't know why that wouldn't include to Amtrak.
ScotteVest I don't have one of these but see their reviews pretty often. I'd probably opt for a fishing vest or photographer vest, though, don't need my pockets to be invisible.