As with most topics, the more you look at it the more you know you don’t know. One company I would love to know more about is Hoffman's.
Hoffman's was a pre-war manufacturer of OO gauge freight car kits in Philadelphia. Their kits included painted and lettered card sides, wood bodies, and cast details, including sprung Bettendorf trucks. The line included:
Box car, RDG, DL&W
Flat car, RDG
Reefer, wood, MDT, ARLX, FGEX
Caboose, eight wheel
Details are a bit sketchy but I did manage to see one of their kits at the home of Bill Chapin some years back. The photo of is the kit and their instruction sheet. If you have more Hoffman's I would love to hear from you.
UPDATE: For the first Hoffman's advertisement from 1938 see this article, and for photos of three more Hoffman's kits see this article.