Signage
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.
Read more about this topic: Washington Metro Signaling And Operation