Washington Metro Signaling and Operation - Signaling

Signaling

Fixed signals are only present at interlockings. They show a red over red aspect for “stop and stay,” a lunar (white) aspect for “proceed,” and a flashing lunar aspect for “proceed on diverging route” (only present on newer interlockings; older interlockings are being updated with signals that show this aspect). Tracks are divided into block sections, which can only be occupied by one train at a time, a method to protect against collisions used by most railway systems worldwide, but there are generally no fixed signals to protect block sections (signals tend to be only at beginning/end of route and just before track switches). Virtual signal aspects are transmitted to the train by a cab signaling system and displayed to the operator in the cab, as well as transmitted to the ATC system. Entry into an occupied block is prevented by an automatic train protection system (ATP), which supervises both manual and ATC operation and stops the train well before it would enter an occupied block.

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.