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margin: 10px 10px 10px 10px; border: 1px solid #666; padding: 0px;" src="https://images.cf.nycsubway.org/images/bsg/sighead.jpg" /> All divisions of the New York City subway use what is now called "wayside color-light block signalling", that is, signals are physically located to the side of the track (as opposed to being overhead, or transmitted directly to the train), utilize the colors of lights (as opposed to semaphores or other devices, or positions or shapes of lights), and rely on dividing tracks into discrete <b>blocks</b> (or <b>track sections</b> or <b>track circuits</b>) to electrically detect the presence of trains. Nearly all of the routes of the current subway are signalled with classic wayside block signalling, whose principles have not changed markedly since the opening of the IRT in 1904. (In-cab signalling and recent position- and radio-based technologies are slowly being deployed on some lines; the first to be cut over is the "L", chosen because it shares no trackage with other routes, and has a relatively simple service pattern, i.e. no express service.) </p><p>Tracks are divided into <b>track circuits</b> (or <b>sections</b>, or <b>blocks</b>), normally hundreds of feet long, by <b>insulated joints</b> in the rails. Trains' wheels and axles short-circuit the two rails to indicate their presence in a block. While every signal occurs at an insulated joint (a block boundary), not every joint has a signal at it. Each block can either be <b>occupied</b> (any portion of a train in any part of it) or <b>vacant</b> (no train anywhere) at any time. Every signal has a <b>control length</b>, defined as the distance, a number of whole blocks beyond the signal (i.e., in the direction of traffic governed by it), whose occupancy forces the signal to <font color="#FF0000">red</font> ("stop") - the control length of a signal enforces that separation between trains. A signal being <b>clear</b> (<font color="#009911">green</font> or <font color="#ffcc00">yellow</font>) attests that there are no trains, or parts of trains, or other obstructions anywhere in its control length, that is, that its control length is clear of dangers. The New York City subway relies on <a href="/wiki/Subway_Signals:_Train_Stops" title="Subway Signals: Train Stops">automatic train stops</a> to "trip" noncompliant trains, that is, force them to obey signals that indicate "stop". Control lengths and signal placements are chosen such that an out-of-control train <a href="/wiki/Subway_Signals:_Train_Stops" title="Subway Signals: Train Stops">"tripped" by a train stop</a> will come to a halt before reaching the obstruction or danger. Control lengths also <b>overlap</b> each other to keep trains at least one signal apart. </p><p>Signals in the vicinity of switches ("switch-tracks", "points" (UK)) must be and are controlled in a coordinated fashion known as <a href="/wiki/Subway_Signals:_Interlocking" title="Subway Signals: Interlocking">interlocking</a>, which usually involves a human <b>tower operator</b> selecting the routings of trains. </p><p>The pages listed below explain more about the various kinds of signals and signal features, and include illustrations of the signals' <b>aspects</b> (distinctive displays of colored lights) and the corresponding <b>indications</b> (what they mean). The discussions are oriented towards those who have spent some time looking at the system from the front of a train, and have already seen, if not fully understood, the signal system's appearances and behaviors. They should be browsed as a set. </p><p><b><a href="/wiki/Subway_Signals:_Approach,_Automatic,_and_Marker_Signals" title="Subway Signals: Approach, Automatic, and Marker Signals">Approach, Automatic, and Marker Signals</a></b><br /> The most common signals, plus train order signals and miscellaneous light signals. </p><p><b><a href="/wiki/Subway_Signals:_Train_Stops" title="Subway Signals: Train Stops">Train Stops</a></b><br /> The mechanism that forcibly stops trains that disregard signal indications. </p><p><b><a href="/wiki/Subway_Signals:_Time_Signals" title="Subway Signals: Time Signals">Time Signals</a></b><br /> Grade time and station time signals that enforce train speed limits. </p><p><b><a href="/wiki/Subway_Signals:_Interlocking" title="Subway Signals: Interlocking">Interlocking</a></b><br /> How signals and switches are controlled together. </p><p><b><a href="/wiki/Subway_Signals:_Home_Signals" title="Subway Signals: Home Signals">Home Signals</a></b><br /> The signals with two or more "heads" near switches. </p><p><b><a href="/wiki/Subway_Signals:_Sign_Signals" title="Subway Signals: Sign Signals">Sign Signals</a></b><br /> Fixed metal signs used in signalling. </p><p><b><a href="/wiki/Subway_Signals:_Holdout_Signals_and_Bidirectional_Traffic" title="Subway Signals: Holdout Signals and Bidirectional Traffic">Holdout Signals and Bidirectional Traffic</a></b><br /> Signals that enforce the direction of traffic on a length of track. </p><p><b><a href="/wiki/Subway_Signals:_Single-Line_Signal_Diagrams" title="Subway Signals: Single-Line Signal Diagrams">Single-Line Signal Diagrams</a></b><br /> How to understand the "single line" diagram format used to express the layout and capability of signals. </p><p><b><a href="/wiki/NXSYS,_Signalling_and_Interlocking_Simulator" title="NXSYS, Signalling and Interlocking Simulator">NXSYS, Signalling and Interlocking Simulator</a></b><br /> A complete interactive simulation of virtually all features of New York City Subway signaling. </p> <h2><span class="mw-headline" id="Page_Credits">Page Credits</span></h2> <p>Descriptions and graphics in this section (unless otherwise noted) are Copyright © 1997-2002 Bernard S. Greenberg (<a rel="nofollow" class="external text" href="https://www.nycsubway.org/perl/feedback22?16">contact</a>). </p>    </div></div><div class="printfooter"> Retrieved from "<a dir="ltr" href="https://www.nycsubway.org/w/index.php?title=Subway_Signals:_A_Complete_Guide&oldid=10357">https://www.nycsubway.org/w/index.php?title=Subway_Signals:_A_Complete_Guide&oldid=10357</a>"</div> <div class="visualClear"></div> </div> </div> <div id='mw-data-after-content'> </div> <div><br><br><br><br><br><br><center> <script type="text/javascript"> </script> <script type="text/javascript" src="https://pagead2.googlesyndication.com/pagead/show_ads.js"></script> </center> </div> <div><br><br><font color=black size=1 face=courier>https://www.nycsubway.org/wiki/Subway_Signals:_A_Complete_Guide<br> Copyright © 1995-2012 nycsubway.org.<br>nycsubway.org is not affiliated with any transit agency or provider.<br> Not mobile.<br></font></div> </div>  </body> </html>