Richard Hendrickson wrote:

... the later AAR recommended practice stock car Ben mentions,
which was based on a Union Pacific design, did not appear in the
Cycs until 1953.

I don't have my notes at work, and I don't know what it was based on,
but I have long been under the impression (delusion?) that the D&RGW,
GN, and CB&Q all had stock cars that were closely related to the
AAR standard, while I thought that UP cars had diagonals running
the other way... But I could be wrong of course. :-)

Tim O'Connor