Grain doors were used for decades when loading box cars with grain. They went across the side doors to enable loading, to minimize leakage past the bottom of the doors, and to enable doors to be opened at destination.
Active country elevators always had a few stacks of grain doors on hand, so if you model one or more country elevators you need a few stacks of grain doors. Go to the item and its associated description for more details about grain doors and how to use them.