Here is a picture of the wheelchair accessible MK train car.
A small ramp folds down from the side so that the wheelchair can be rolled in.
The guest sitting in a wheelchair will be facing forward; at least one member if your party will sit across from you, facing the rear of the train. If there are several wheelchairs or you party is larger, the rest of the group will be asked to sit in another train car.
Although all the train stations are accessible, I would recommend the one in Fantasyland at Fairytale Circus. The entrance and exit at that stationare fairly flat and there is a large seating area where you can wait for the train.
The Main Street station has a very steep ramp with several switchbacks. The Frontierland station is not as steep, but still has several switchbacks and does not have much space for waiting.
If you are planning to go to AK, she might enjoy the wildlife train to Rafiki's Animal Watch. That train is accessible to both wheelchairs and
ECVs. Here is a picture of the accessible car on that train.
Wheelchair or
ECV users back into the train and all passengers ride facing sideways.