My sister also has a walker. On Viewliners, there is space at the end of the car to store them. Or if they fold up, and the train is not full and you don't mind sitting apart, they can be fit between the seats. On Superliners, you may consider getting a lower level seat or room. This way, you do not have to carry it up the stairs!
And if you do get a room on either train, I'd recommend getting a bedroom. There is extra space to store your walkers. Roomettes do not have extra space for them!