Minecraft wiki entry for Detector_Rail, associated text: When placed, a detector rail configures itself to line up with any adjacent rails (including activator rails, powered rails, and other detector rails), as well as adjacent rails one block up. If there are two adjacent rails on non-opposite sides, or three or more adjacent rails, a detector rail lines up in the east-west direction. If there are no adjacent rails, a detector rail lines up in the north-south direction (but if a rail is later placed to the east or west, the detector rail re-orients itself in the east-west direction even if it is already connected to another rail to the north or south). If a rail it would line up with is one block up, a detector rail slants upward toward it (with multiple options to slant upward to, a detector rail "prefers", in order: west, east, south, and north). Other configurations can be created by placing and removing various rails.