At switches entering into pocket tracks in a direction reverse to the normal operating direction, there is a "P Signal". The P Signal is a single lunar signal that displays the letter "P" and is found at the entrance to some but not all pocket tracks and other selected main line interlockings in the reverse direction of the normal flow of traffic. Below the P signal on the same post is a punch box to allow the train operator to manually set the switch to the diverting direction (reverse) when the P Signal is lit. The P signal is only lit when a train occupies the two track circuit approaching the P Signal and all possible routes into the interlocking are not occupied.