My principal source of information is the compilation of "1937 AAR Boxcars (1936-1947)" by Ed Hawkins, which lists 300 RDG cars, numbered 103000-103299, built in Nov 1937. They had Duryea underframes, 3-panel Creco doors, Alan Wood Super Diamond running boards, and an unusual side-sheet rivet pattern. That would make for a quite unusual model, but I probably won't be ambitious enough to tackle it.
I appreciate your suggesting K4 Decals for NKP, and I have been corresponding with Michael Hood. I would like to model a car from the #15500-15999 series, built in Sept 1937, as it would have looked in the first half of 1941.
The decal sheet that K4 currently offers (in several scales) uses a version of the NKP herald in which the 'K' in "Nickel Plate Road" has a curved "tail" but the 'R' does not. Do you or anyone else happen to know when the decorated 'R' came into use? Pg 146 of Ted Culotta's "Freight Car Reference Book" shows that herald on NKP #20177, which was built in January 1940. However, in that photo taken on July 10, 1943, the car looks as if it has been freshly repainted.