I'm resurrecting and commenting here, because this thread has been referenced in another current thread, and I think I can add some more information to the discussion.
In the photos above, I believe that the "unidentified" boxcar, second from the largest, is a Frank's Roundhouse item, based on one of the many FRH series' ancestors: Auburn/Kris/AMT/KMT. etc molds.
Bigger than a 6464, but not quite scale-sized.