Minecraft wiki entry for Minecarts, associated text: Another method to check for a rider is to take advantage of the change in the speed of the cart, as an empty cart slows down more quickly. In this design, a minecart creates a pulse when it passes the detector rail. If the minecart is empty, it gets to the turn just as the signal does and be sent back. If the minecart has a rider, it gets to the turn before the signal and be able to continue on. This design requires the track to be set by default to let the cart through.