When I was younger and skinnier I could get up the stairs with forearm crutches. They fold so I can carry them on my wheelchair or in luggage to get to the train unassisted. A standard folding walker with skids, not wheels, would work for someone who is good with it. In tight areas, you wrap the walker around your body with the handles front and rear, and hold the handle in front and the crossbar now on the side. For the stairs, fold the walker so it is easy to carry in one hand and use the very solid banister. The vestibules between cars are the trickiest. Move one body part at a time: hand off walker and on grab bar, other hand moves walker forward, move one foot to solid plate, move other foot, etc. It is easier than getting off those moving walkways airports have.
As to transferring to the cafe car on superliners, I did it once. You can only transfer at stations where the train stops for several minutes. You can eat (and pay for) cafe food but not get your included sleeper meal there. The cafes do not all have operating restrooms and they are not accessible anyway. If you want to try this, ask your SCA when you board to set it up. A scenic section is best because the cafe windows are so much bigger than the H-room windows.