coal drop

noun1

1. •
   An elevated railway track designed to allow material to fall freely between the rails onto the ground beneath. It is used to rapidly unload hoppers containing coal and other bulk materials.

   • The ladies remained at the house, while the men walked to the staith on the [River] Wear, and were shown the process of unloading the wagons into the keels by means of the coal-drop. — 1948