I recently bought Bossman Games' Black 5 and I want to utilise the different braking modes for fitted stock both with and without D.A. valves. My question is how common was it for wagons to have D.A. valves? Was it standard across most fitted wagons or was it only router login on certain types? Was it a later development or was it standard by the 1930s? Did trains ever run with a mix of D.A. and non-D.A. stock? A google search reveals plenty of information on what a direct admission valve is, but shockingly little on it's actual implementation, hence why I ask here
