Pictures:

The original photo (33k) was found in a Märklín catalog, although I have liked this car since I was a kid.

Side A (19k) - A side view of the freight car with the sliding door 1/2 open. Note that the door is fixed and doesn't slide at all.  The door is made from a stack of technic beams and 1xn tiles held in place by technic pins that have a stud on one end. Also there is a fake floor to hide the train plate holes and a interior facade that hides that backside of the technic beam stack from the inside. Also notice the detail on one of the trucks. I was trying for something new.

Side B (19k) - A side view of the freight car with the sliding door closed. Note that the door is fixed and doesn't slide at all. The black door tracks are just plates.