Minecraft wiki entry for Detector_Rail, associated text: Detector rails (and other rails) act as "roads" for minecarts. A minecart that enters a detector rail's space from either end of the detector rail continues to move, losing only a little velocity (which can then be increased again with powered rails). A minecart that enters a detector rail's space from the side turns east or south (depending on the detector rail's orientation), or in the downward direction for a slanted detector rail.