I would guess that the sliding doors worked great for about 15 minutes, then got all gummed up with dirt and coal dust in the summer and ice and snow in the winter. Unless the only slide track was on the top, and the bottom didn't have a track.
One of the books (probably Two Feet to Tidewater) had a quote from Earl Keefe about the last train, in #8. He said that the engine had the "rat trap" door, which had the seat on the inside of the door. In order to get out, you had to stand up, move the seat, and only then could you open the door. I think that feature was long gone on #9 by the time it got to the KC.