Funny you should ask as I just completed thatvery trip and it is a great way to see the vastness of Canada.
I was sble to get some picturesand ask some questions. i am elderly but still ambulatory and could see no way that someone in a wheelchair could get around on the train as aisles are very narrow so I asked around as I thought I saw domeone board in a chair. I did see herbut apparantly she can walk a bit
This is what i eas told. If you are travelling couch there are two spots per train for those wishingvto remain in their wheelchair. These spots are quite close to the accessible washroom. In couch no meals are supplied  and you would either bring your own or get someone to get you something
In sleeperclass there is a room that is accessible but was not on the train i was on so I coukdnt take any pictures. The train car door on that train is right outside this room and you enter right into the room. It is quite rfespparant and has 2 beds and washroom and shower As you cannot get to dining area all of your meals are served in your room. Seems a little sad as the best part of our trip was meeting new people There is a very large window for viewing
Here are the pics