Quality Craft kits came out starting in the late 70s. They consist of metal castings, assorted hardware, decals, and pre-cut / color-coded / polished wood (trucks and couplers not included). The same kits were later sold by Gloor Craft. These particular kits were sold in 3-packs.
I'm told that this model is likely based on cars used mainly by Eastern logging roads in the late 19th / early 20th century. They did not catch on with Western loggers, who were busy developing their own methods. These cars had a light railroad rail on each side, upon which a loader equipped with flanged wheels would move. The loader would ride on the first car to the loading site; then be fired up and move to the second car in the train, where it would stay while it loaded the car it had just vacated. As each car was loaded it would move to the next car behind until it reached the last car. At that point the loader would be secured and ride that car back to the mill.