Minecraft wiki entry for Activator_Rail, associated text: If there are two adjacent rails on non-opposite sides, or three or more adjacent rails, or no adjacent rails at all, an activator rail aligns itself in the direction the player is facing. If an existing activator rail is connected to only one other rail or none at all, and a new rail is placed perpendicular to the activator rail, then the activator rail aligns itself in the east-west direction (if it isn't already facing that way). If a rail it would align with is one block up, an activator rail slants upward toward it (with multiple options to slant upward to, an activator rail "prefers" west or south). Other configurations can be created by placing and removing various rails.[needs in-game testing]