Minecraft wiki entry for Clock, associated text: When running an empty minecart on the loop or back-and-forth, the cart generates redstone signals as it passes over the detector rail(s). Minecart clocks can be extended or shortened easily by adding and removing track, to adjust the delay between signals. On the flip side, they are easily disrupted by wandering players or mobs, and a long clock can take a fair bit of space. Also, the exact period is generally not apparent from the design. The need for gold in the booster rails can also be a problem for some players.