Minecraft wiki entry for Storage_minecarts, associated text: Design A is probably the simplest. As soon as the hopper receives at least one item from a cart that arrived above, it deactivates the Powered Rail so the storage cart that came from the left will be stopped on top of the hopper. After taking all the items in the cart, the powered rail gets reactivated so the cart starts going left. A possible downside of this design is that the powered rail is always active as long as the hopper is empty so if a cart approaches the unloader too fast, there is a non-negligible chance that the cart bounces off before getting braked by the deactivated powered rail. (This will also happen a lot if you use a second hopper below the one doing the unloading.)