Just returned from WDW last weekend. Took the Auto-train down and back. On our trip down, we were basically on time until right outside of Sanford--had to wait for tracks to be cleared, and unloaded c. 30 minutes late as a result. On the return trip to VA, we actually arrived an hour early. Departure both trips was slightly early/ahead of schedule (because all passengers and vehicles had been successfully loaded). I believe some of the delays that occurred earlier this Springtime were a result of the situation with wildfires in GA/FL. We did see at least one burned out area where fires had advanced right up to the train tracks at some point.
There are wheelchair accessible accomidations not only in sleeper, but also in coach. In both cases, they are located on the lower level... and the staff will bring meals to your car for everyone in the party (since dining is located on the upper level, and one would need to walk from car to car on the moving train in order to get to the dining car). In coach, the wheelchair accessible restroom was just a short distance from the seating area (right next to the coach doors, and just across from the private seating area). Sleeper is probably more comfy, but I wanted to let you know that coach was also available. Special needs passengers also load and unload first, so that you don't have to worry about dealing with lines of other passengers.