Minecraft wiki entry for Storage_minecarts, associated text: Design A is probably the simplest. When the hopper has at least one item, it deactivates the Powered Rail so a storage cart coming from the right will be stopped under the hopper. After feeding the cart with all the items in the hopper, the powered rail gets reactivated so the cart starts going right. 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 comes to the empty loader it immediately bounces off in the opposite direction. This might lead to the inefficiency of the system if there is a great distance between the loader and the unloader.