The Guidebook for Guests with Disabilities and the park maps list the MK train as something where an ecv rider would need get out of the ecv and leave it at the station. That means a round trip, either getting out and riding in the train seat or switching to a manual wheelchair. The Conservation Station train at AK is listed as wheelchair and ecv accessible.
The park maps have little symbols to tell if it is completely accessible (pic of wheelchair), accessible for wheelchairs only (pic of stick person moving from an ecv to a wheelchair) or you must get out for the ride (pic of stick person getting up from wheelchair).
Andrew makes a good point about the SIZE of the ecvs. I read a post from some Universal Studios CMs where they explained why ecvs are not allowed on some rides where power or manaul wheelchairs are. There are many variations in the size of ecvs; how wide they are, how long they are (the "footprint":size of the space they take up). They indicated trying to get all the ecvs to fit the rides would be almost imposssible because there is so much variation. One ecv might fit and another might not, so it's easier just telling people with ecvs they can't go on. Most ecvs tend to be owned and rented by people who are ambulatory, but maybe unsteady or can't walk long distances, so the assumption is that someone using an ecv could get out for the ride.
Wheelchairs (either manual or power) tend to have fairly similar footprints. In the US, 2 major brands account for most of the manual and power wheelchairs sold (Quickie and Invacare). So, it is easier for those wheelchairs to fit in the rides. Most people with power wheelchairs are not able to walk at all or at least not well enough to board rides, so without their wheelchairs being able to get on the ride, they can't go.
