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