The monorail is accessable. (Except for the driver's compartment.)
The train running last week (The Roy O. Disney I think) could only load/unload Wheelchairs / ECVs at New Orleans or Tommorowland. Toontown has a handicap ramp but for some reason it dosen't match up with the train's handicap car. (The very last car on the train.)
Main Street station has no handicap ramp at all. They will allow you to leave your scooter/chair at any station and make a full circuit.