One of the more interesting early OO gauge models that I have only seen a few of over the years are the cast aluminum passenger cars by Nason Railways. Introduced in 1934, they were joined in their line by the easy-built (later called "eazy-bilt") versions by 1936, which implies these were perhaps not so easy to build. Both versions were available until WWII.
Dick Gresham was kind enough to send photos of the entire line from his collection. These two are representative. The line included a PRR P-70 coach, a PRR PB-70 coach-baggage, a 12 section Pullman, and an "all-service express" car. This first photo is of a built up version of the all-service express car. Looking at the photos this model has always looked rather free-lanced to me, but perhaps there is a prototype somewhere. I am not sure which style of truck is on this model, but Nason listed three types of four wheel passenger trucks in their catalog (PRR plain, PRR roller bearing, and Commonwealth).
The second photo is of all the parts in a kit for a Pullman. The floor, sides, and ends are sand cast aluminum, with a wood roof and other details including bronze couplers. This car is equipped with their six wheel Pullman trucks.
These cars are pretty recognizable, there is nothing else like them really with the sand cast sides and frame, and from what I can tell they seem to be somewhat uncommon. Someday I hope to own one, but for now I will have to content myself with a couple of the later version of their passenger car kits with brass sides.
UPDATE: Dick has come through with a couple more photos of interest. First, the underside of the all-service express. I find it interesting how this car is described in the 1940 (6th edition) catalog as "a unique car, different from the usual run."
Second we have a close up of the two and three rail versions of the PRR plain four wheel passenger truck. The all service express car in the photos is equipped with the Commonwealth truck. Dick also reports that he sees two versions of the 6 wheel truck in his collection. I see no note as to two versions of this truck in the 1939 or 40 catalogs, this may reflect some evolution in the design during production over the years.
UPDATE 2: One more photo from Dick G., it shows both versions of the Nason 6 wheel Pullman truck and also another view of a built up Nason sand cast Pullman.
UPDATE 3: Another example of the Pullman may be seen here.