This "Red Ball" kit car first appeared in the late 70s / early 80s. Based on the picture of the unassembled kit below, I'm guessing it's either resin or plastic (as opposed to the mixture of metal castings and wood parts normally found in these kits). As pictured, trucks and couplers are not included. Advertisements back in the day listed both "Trussrod" and "Fishbelly" versions of this model. However, I'm not sure if the one kit could build either, or if there were actually two different kits. But honestly, at this point in history does it really matter?
I don't know what specific prototype this model might be based on (if any), but it looks a bit like the composite gondolas designed by the USRA during World War I.