I honestly don't know the dimensions of the stairway, but I've seen people drag some pretty big suitcases up those stairs. And if you can fold the walker up, then I can't imagine how you wouldn't be able to get it upstairs.

All that said, the walker should be perfectly safe in the luggage rack. It's very rare to hear about anything being stolen from those racks, much less something so big.

Finally, there might even be another alternative for you. Each Superliner is equipped with an outside compartment for carrying larger things like skis for skiers heading for the slopes. That compartment I believe is locked by the attendant making it rather secure. Even if it's not lockable, one certainly can't access it while the train is moving and so few people even know that they exist, that the potential for theft is minimal even when the train is in a station. I can't promise that this locker is large enough, but it might be.