I am guessing that the 50’ Double-Plug Door Boxcar is the Evans. That is an RBL. RBLs primarily carry canned food products. I have also heard of them carrying paper, paperboard/cardboard, plywood, and tin coated steel (not common). They could also be used to carry anything that can fit through the door opening.
The PS 5344 boxcar, an XM type boxcar, would carry similar products as the Evans RBL. I would guess single door XM boxcars most often carry the lumber/paper products though. They do carry tomatos in drums and bag-in-box from tomato processing plants out here. I have also seen them carrying composite roof shingles from a local plant. According to a website that lists waybills (I cannot attest to the validity), they have also carried toys.
The 54’ PS covered hopper would be used for grain, soda ash, potash, fertilizer and other light granular materials. It wouldn’t be used for cement or sand nor would it be used for plastic pellets. The Athearn 55 ft ACF covered hopper is a plastic pellet or resin car.
The 50 ft double door car would carry lumber, autos (1930’s-1950’s), paper, appliances, auto parts and pretty much any commodity caried by a boxcar except grain.
The PS 5344 would carry pretty much anything carried in a boxcar.
Is this http://www.athearn.com/Product/Default.aspx?ProdID=ATH91363 the type of car you are refering to? If so, its most common commodity is probably canned food products. It appears that lumber companies leased these cars back in the 70s, so lumber products were probably fairly common loads back then.