On Aug 26, 2009, at 6:53 PM, Rob Adams wrote:

I'd wager that it is a former C&NW car built in the 1920's. See Jeff
Koeller's article on p. 42 of the September 2003 issue of Mainline
Modeler. I believe that Sunshine has produced a kit for these cars in
HO. See their kit #49.1.

Rob, I'm sure you are correct. I hadn't considered the C&NW cars
because they were not as tall as the USRA cars, and the proportion of
length to width is hard to judge in the photo. But it turns out I
have a Soph Marty photo of an identical C&NW car - maybe even the
same car - slightly less weathered and with the same white board
above the reporting marks, which doubtless carried special assignment
stenciling.

Richard Hendrickson