These cars date back to the early days of N scale (late 60s / early 70s). They had various importers over the years (Revell and Walthers, most noteably). They were available either unloaded, with two containers, or with two trailers. Con-Cor also imported these cars and sold them in paint schemes not available directly from Arnold.
I don't know what specific prototype this model might be based on (if any), but generally speaking, flat cars like these first entered service in the 1960's. They were designed to accommodate the latest 40' trailers that were just being introduced on the nation's highways. Each flat car was set up to transport two of the trailers. Subsequently, these dedicated TOFC (Trailer On Flat Car) flatcars became the defacto standard for intermodal service during this era.