One major source of prototype freight car information I use for modeling is RAILWAY PROTOTYPE CYCLOPEDIA (RP CYC). These books, 35 volumes in all, are long out-of-print.

I made a handy downloadable RP CYC index by subject:

rp-cyc-index-by-subjectDownload

But, obtaining RP CYC issues can be very expensive as they are highly desireable.

There is a much more cost-effective way to get similar info: the old Mainline Modeller (MM) magazine – a digital version can be obtained from the C&O Historical Society (https://chessieshop.com/index.php?main_page=product_info&cPath=74_121&products_id=3291). I think this is by far the best value for really good prototype information, written by those who knew the subject well.

I have made a downloadable index of MM freight car articles for the 1935-1945 era:

1939-45 Era – Mainline Modeler – FreightDownload