This was a common practice at grain elevators, especially when harvests were heavy and covered hoppers for grain in short supply.
In my area, Southern used to have 40' and 50' box cars with roof hatches primarily used in kaolin service. Near my house, there's a feldspar mine that utilized 50' box cars on several occasions to ship bulk feldspar. Only saw that occur once or twice and these cars had paper stretched from side to side at the door opening. For the most part, bulk moved in covered hoppers and bags in box cars. Now it's over the road and double stacks.
