There are heavy-duty wheelchair lifts that climb stairs (really a heavy banister rail) and can navigate turns and fold away when not in use, but I think turns require a fairly large landing for the transition. I used one at the NY Transit Museum in Brooklyn. They can even fold and unfold automatically, and be operated by the occupant on the lift.This would be woefully insufficient. Those only work for people with partial mobility problems who can generally move themselves. Imagine a large, full adult with a battery-powered scooter. Both the scooter and the adult need to get upstairs, and they could weigh 200+ lbs each. Transfer is possible with two staff assisting (see what the airlines do), but it takes time that planes have during pre-boarding, and which trains generally do not at a short stop. Air travel also does involve moving around the plane other than to and from the bathroom as a part of the journey. Scooters are also often quite large, and would not necessarily be able to fit up the staircase pathway.
But they are very slow, and the stairway needs to be closed off to pedestrians while the lift is in use. At the Transit Museum, it was installed on a separate entrance that was only used by staff and lift users. I don't think one would be practical on a railcar.
Here is a web page for anyone who would like to know more:
Inclined platform lifts or wheelchair lifts help provide barrier-free accessibility for the disabled in homes, businesses and public facilities.