Of course this only works with cars that are designed for the purpose - at one end is a standard coupler with a square shank, the other has a round-shank coupler - thus as long as a string of cars is coupled round-to-square throughout, it can be unloaded in such a mechanism. How to tell one end of a car from another? The ones with such a coupler arrangement are marked at one end by a couple or three feet being painted a bright colour - usually yellow, orange, or red.
The car doesn't have to go completely around as implied by the design above.. only far enough to disgorge its contents, then back up.
Another option might be to build some kind of mechanism that would reach under a hopper to unlock then open its doors - emptying the hopper through some tie-less track. Then it would reverse the process closing and locking the doors. This all wouldn't necessarily have to be computer controlled, it could be manually activated by an operator standing at the side of the layout. It'd be a big enough trick to build such a mechanism, let alone make it autonomous.
Instructions to build an 8-wide hopper car are pending.
