Washington Metro signaling and operation

Signaling and operation on the Washington Metro system involves train control, station identification, train signaling, signage, and train length. As with any working railroad, communication between train operators, dispatchers, station personnel and passengers is critical. Failures will result in delays, accidents, and even fatalities. It is therefore important that a comprehensive signal system operated by a central authority be in place. This gives individual train and station operators the information they need to safely and efficiently perform their tasks.

Fixed signals are only present at interlockings, and consist of three lamps: two red and one lunar. 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. Similar signals are also in place on the Baltimore Metro SubwayLink, the PATCO Speedline, and the Miami Metrorail.

Two trackside signs exist: The "S" Sign is posted at revenue tracks and indicates to the operator that a station is ahead. It is located at a distance no greater than 1200 feet (365.75 m) before a station platform. The "TB" Sign is posted to indicate to the operator that an eight car train (the longest used in the system) is clear of an interlocking ("turnback") when the train front is flush with the sign, and that the train can turn back in the direction from which it came on the opposite track.

When it first opened, Metro ran a mix of four and six car trains; however, there have not been any four car trains used in the system (except for the money train) since the mid 2000s. The most common train length is eight cars due to the fact that the 7000 series railcars can only be operated this way. During peak hours, a mix of six and eight car trainsets are used, while during off peak and weekend hours almost all trains are only six cars in length. In 2008, Metro began planning to acquire sufficient quantities of the new 7000 Series cars to run 100% eight car trains. Metro also tested eight car trains on the Yellow and Blue lines in preparation for increased service during the 2009 inauguration of Barack Obama. However, by early 2010 riders began to lose faith in WMATA's "promise" to implement 100% eight car trains. In December 2010 those fears were confirmed and Metro delayed the plans indefinitely. as of 2012, during peak hour service the Red Line runs 51% eight car trains, the Orange Line runs 40%, and the Green Line runs 50%, while the Blue and Yellow Lines run only six car trains, for a total of 43 of 129 trains running at full length, or 33%.