I think I remember seeing one or two of automobiles tarped on flats, but I've never seen a picture from the 20s, or any time, of a vehicle being unloaded from an end-door boxcar.
Yet another guess, that D&RGW went to flats instead of boxcars as vehicles got bigger - having been in the outfit cars (converted from 4000-series boxcars), getting a 1940s-vintage Ford or Chrysler secured into a boxcar would probably have been interesting. And I would think many vehicles would have been too tall, too.
On the standard gauge, I would guess that a regular XM boxcar would be used. In the 1950s or so, Southern Railway experimented with a couple of different ways to haul automobiles. They had "all door" boxes with sliding-door-like sides, designed to allow cars to drive in from a loading dock. And they had at least one boxcar with door/ramps hinged at the bottom. The door would swing down, and the car would be driven onto it and secured to the door. It would then be closed, and the end result was several automobiles, standing on end, roof-to-roof inside the boxcar.
Double- and triple-decker flats killed the boxcar, of course.
JAC