Minecraft wiki entry for Storage_minecarts, associated text: Design C has the advantage that minecarts with chest will not get stuck as long as they are empty while entering the loader. This design has two operational modes: when the loader doesn't have a cart, a small chest that serves as a "buffer" will accumulate items from the large chest above. And when it has a cart, items in the buffer will be transferred to the cart and nothing more. It has a T Flip-Flop that switches between two modes, connected to a detector rail.