Fully Enclosed racks became the norm in the Mid-late 80's. Before that, roofless versions(70's), and even more before that, open racks (60's).
In the manner CN had developed in the 1950s, in the 1970s other North American railroads began refining their autorack cars. They began installing side sheathing to protect the vehicles from impact. Roofs were added to most autoracks in the 1980s, and end doors were added in the latter portion of the decade (both to prevent damage and to deter people from boarding the cars and riding the train within the vehicles loaded in them).
see the pic rock probably used it in pool service?