Minecraft wiki entry for Minecarts, associated text: Depending on orientation, the signal may need to be inverted with a redstone torch. The torch adds a minor delay, but this shouldn't affect the results. This design assumes the minecart is moving at top speed. If your minecart is moving slower than that, you can either add delay to the wire using repeaters or increase the speed of the minecart with a booster just before the tripwire.