Thanks John. I have modelled several end dump wagons with the door left partially open by one of the tipping gang. Similar end tipping operation was used here at coal shipping docks but using a hydraulic ram rather than steam. At some docks they used to empty side door wagons the same way, with an off centre ram.
In the 'old days' they used a bascule type 'tipping/discharge engine'. Wagons were pushed out directly over the ship's hold on a short section of track mounted on 2 arms. The weight of the loaded wagon would tip the bascule arms down and simultaneously raise weights, which in turn would overcome the weight of the empty wagon & return it to the staithe (pier) height track. The idea was to reduce coal breakage by reducing the height of the fall into the ship's hold. Way before (even !) my time though.