Doug,
I believe there was an article in NG&SLG on the D&RGW early 4 wheel cars. I also seem to remember that there were drawings as well. I don't remember if passenger equipment was included, but the freight - flat, box and gondola- were.
The BHI Publications book on D&RGW passenger equipment begins with a standard 4 axle 30 foot coach.
Interesting project, I will follow out of curiosity. 1:20.3 modeler here.
Bob c.