The train is wheelchair accessible, but not
ECV accessible. (Although I have seen some very small
ECVs a very few times).
Guests using ECVs can bring it to the train station, then park it there and take a round trip. If you can't step onto the train, you could switch to a wheelchair, available at the station.
I'm assuming you mean Enchanted Tales With Belle by ETWB.
That attraction is wheelchair accessible, but not ECV accessible. You would park the ECV with the strollers. If you would not be able to walk in line, talk to the CM at the entrance before parking the ECV to get a loaner wheelchair.