These open frame auto carriers can be made out of laser cut aluminum and brass sheets and shapes, with tabs and slots for assembly into a 3 dimensional freight car.
You are correct , I have drawings to do just that . The catch is cost . One) brass sheet , 12inch x 24 inch will make 2 sides and 2 floors ,
two) time it takes to assemble them
three) laser time to cut out is figured by the inch , my experiment was aprox $60-$80 for cutting time for just one car (all pieces ) .
Now comes to needing a flat car , Atlas is 89ft , I found most of these real cars were 86ft . So you need to build a flat .
Another thing to consider many roads built their own racks in their shops . See below ( Santa Fe built their own racks for existing flat cars I think called F85 or something like that )
I have built 10-15 of these from styrene with hinged ramps , cushioned underframe . I made special wood "jigs" that slide in from the end and keep all floors square and perfect height . On some the rack uprights actually bowed out slightly at where they attach to the flat then went up to the floors ( see attached photo) . I am not bragging but I took these to a show in south Florida about 13 yrs ago and sold all 10 with "AMT 69 Mustangs , Torinos , Camaros , Chevelles which also were assembled and painted and side windows added . I got $8000.00 for the 10 cars . I probably made 4 bucks an hour when I built them but I was new to O scale 2 rail and was using old Model Railroaders and Mainline modeler Magazine articles and drawings . Here are some pictures of partially built car with brake rigging , the AMT plastic kit cars , Mustang in yellow , Chevelle lower . I lived in Detroit in 1969 and I saw hundreds of these so when I got into O scale I was like you guys , I wanted these cars so I made them as close to scale as I could . Each car held 12 or15 AMT cars , and technically the hubcaps were in the trunk . I cast steel plain rims to replace the chrome ones . I was insane back then , could sit for hours working on these things .Sadly I never took any photos of the 10 cars as I had took them to the show , just for show to get scratchbuilding jobs . The guy who bought them as they say " made me an offer I could not refuse" so I took the cash and he got the cars and I was so overwhelmed by all those $100 bills I forgot to take a picture when he took them . This UP car was for a fellow in Ware Mass. if I recall . He bought many many 86ft resin cast high cubes from me . I was making them long before Lionel , by 10 yrs.Yes it would be cool to see someone make these but if you add the correct detailed autos with no hub caps , windows up and price stickers on the side windows , even made in China , if done correctly I would guess $300 per car and to sit at the correct height I do not think you could do 3 rail because the flanges . However a lot of the newer racks have 28 inch wheels , I do not think that any company will invest for tooling and when you add in all the car variations and the giant clan of nitpickers well ......... fa getta bout it !!!!!!!!!!!!!!!!!!
https://www.youtube.com/watch?v=11NPB9ApF2E you tube link