Minecraft wiki entry for Storage_minecarts, associated text: Design D will detect if a Minecart with Chest or Minecart with Hopper is full, before sending it back to where it came from. It will only return a full cart. The minecart is stopped by an unpowered, inclined Powered Rail with a Hopper above. The minecart will rest on both the Powered Rail and a Detector Rail with a Comparator checking if your minecart is full. This design works on the principle that a minecart can pass an activated Piston when going uphill, but cannot pass it going downhill. When the minecart is full, the Comparator is triggered, sending a signal to retract and extend the Piston just long enough to allow the full minecart to pass. Set up your rail so the minecart is not going too fast, or it will overshoot the Detector Rail and not return to your unloader.