This is the "Gravity Unloading Bridge".
Laterly it was only available as part of a set including a special operating hopper wagon, and incline / high level piers and receiving bin with "mineral" (Plastic granules).
It was a Tri-ang Railways product. I think your example has Super 4 type rails?
Tri-ang made several working hopper wagons.
Transcontinental Bogie Hopper (Various Colours)
Transcontinental Bogie Cement Hopper with removeable roof Early light blue/grey, later grey.
BR Hopper Wagon. Grey.
BR Grain Hopper (Hopper wagon with removeable roof with sliding doors. (Early Green, later Grey)
Private Ovwner Hopper Early Red Oxide, later Green.
Hornby Railways re-issued the set at least twice. One set has two Super 4 to System 6 track converter rails, the last issue has System 6 rails on the bridge..
As the special wagon is pushed (or pulled if the loco does not contact any part of the bridge) from right to left in your photo, the projections ( on the bottom side in your photo) catch the retaining lever, allowing the bottom door to swing open, and the load (Usually Plastic Granules) to fall into a receiving bin or another wagon on a siding below....
Also see here...