Minecraft wiki entry for Minecarts, associated text: Powered rails propagate power to each other if they are adjacent and part of the same track, for up to 9 blocks from the power source (1 being powered directly which is propagated to 8 adjacent rails). They also receive power from any adjacent detector rails (when a cart passes over it), even if they are not part of the same track (which follows from the rules above). Because the detector rail powers attached rails, it could be used to activate power rails only when necessary: