It doesn't really disappear. The car has a false bottom. The coal load drops with the false bottom when you push a switch, which makes the level of the coal drop and, from the side, the car seems to be empty. You press the switch again and the floor raises so you now see the full load of coal from the side. If you look in at the car from the top, all that happens is the coal level rises and falls. It's just a gimmick that doesn't have much relationship to real railroad operation.
