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NYC's subway signal system: How it keeps trains from colliding and other things you probably never knew

Like Harry Potter’s Hogwarts, the New York City Transit Signal Training School is hidden away from mere mortal commuters -- only an out-of-place working traffic signal marks its beige entrance at the 14th Street and Eighth Avenue subway station.

It is here that the electrical wizards who maintain the critical signal system are trained. That system, which helps operators to detect where subway trains are over the 659 miles of track in the transit network, includes 12,000 signals, 2,600 track switches and 300,000 signal relays. Over an 18-month course, trainees learn how to troubleshoot and replace relays wires, levers and other equipment, much of which dates back to the Great Depression.

Most trainees have some kind of background in electrical engineering or an interest in electrical mechanics. They are the men and women who, if it weren’t for them, trains might crash into each other throughout the city’s subway network.

On a recent tour of the school, amNewYork heard from top officials about how the signal system works and how critical it is to keeping straphangers safe. Here's what we learned.

1. Signals are used to detect where subway trains are

The most critical role that signals play in
Photo Credit: MTA / J.P. Chan

The most critical role that signals play in the subway system is to detect the whereabouts of trains and to safeguard against collisions. "We need to know where a train is. We need to know in many respects what direction it's traveling. We need to know what train is ahead of that one," said Wynton Habersham, vice president and chief officer of service delivery at New York City Transit, who led the tour of the school. In the above picture, an MTA employee works on signals in the Montague Tube on April 30, 2013.

2. The signal system largely relies on 1930s technology

Perhaps the most important components of the system
Photo Credit: Cristian Salazar

Perhaps the most important components of the system are its electromechanical relays -- basically, switches -- that are connected to tracks. Upon traversing sections of tracks where relays are located, the devices send signals to the system informing it of where the train is located. Many of today's relays go back to the 1930s. But there are various versions, some more modern than others, and the transit agency is slowly moving toward more modern relays. However, the agency says that because of a "robust signal maintenance program" even equipment that is 80 years old is safe even if difficult to maintain.

3. The most at-risk components are the aging cables

Back in the 1930s, when the electromechanical relays
Photo Credit: Cristian Salazar

Back in the 1930s, when the electromechanical relays were installed, the copper cables that were installed to transmit the signals to the traffic lights were insulated with cloth as opposed to modern day cabling that use rubber. Those are still in use today. But water, dust and other elements can cause the cloth to deteriorate. "It can potentially cause leakages in voltage across the wire," Habersham said. The biggest concern is that a deteriorated cable would send a "false clear" -- a signal displaying a "proceed" to a train to go forward even though another train is ahead or a switch is not in the right position.

4. The traffic lights are largely automated

The traffic lights work just like those for
Photo Credit: Flickr / Gene Han

The traffic lights work just like those for cars: red means stop; green means go. However, they are automatically controlled by the movement of the train. As it moves along the tracks, relays do the work of sending the signal of their location along to the lights. A red signal is maintained until a leading train has moved a safe distance. However, some locations, like at Canal Street, Hudson Terminal and others, operators have the ability to manually operate the signals.

5. No failure allowed for subway signals

If a signal fails, the signal will go
Photo Credit: Flickr / Alex Proimos

If a signal fails, the signal will go to red and the train will stop. "We have to treat, you know, the slightest problem, or impediment if you will, as an unsafe condition," Habersham explained. Until it is investigated and resolved, that signal will stay red; trains will be moved slowly, if possible, along the route.

6. The future signal system is operating on the L line

A new computerized signaling system called communications-based train
Photo Credit: Flickr / Stephen Rees

A new computerized signaling system called communications-based train control, or CBTC, began operating on the L line in 2005. While CBTC does exactly the same thing as the current system, it does it in a much more reliable and more efficient way. Using CBTC, the agency can control the speed of the train at every second, know where it is and the direction it is traveling, all at a greater degree than a conventional system. That allows trains to operate more closely together, Habersham says. "You can fit more trains into that same pocket of real estate." However, CBTC is an expensive retrofit that won’t be expanding any time soon; the No. 7 line is next to receive the technology by 2017 at a cost of around $750 million.

7. Subway signal students get to practice with museum-worthy equipment

Some signal classes are held in a room
Photo Credit: Cristian Salazar

Some signal classes are held in a room facing a series of antiquated subway interlocking switch and signal control boards that are still used in the system today. Levers on the control boards operate a fake set of tracks, where students can see how their actions trigger track switches or signals.

8. Subway signal students also learn using a model train

They have the cutest model train at the
Photo Credit: Cristian Salazar

They have the cutest model train at the school! Used to demonstrate and practice the signaling system, the train is somewhat grumpy and takes a bit of encouragement to get running. But watching it go through the traffic signals gives students a chance to see visually how the system actually works. It's like an actually functioning subway train system, just shrunk down to toy size. The model is so cool they should sell it as a kit to make some extra revenue -- the MTA needs all the money it can get.

9. Ghost trains haunt the subway system

Using that cute model train, Habersham showed how
Photo Credit: Cristian Salazar

Using that cute model train, Habersham showed how a track circuit failure can make it appear as if a red light has been triggered by a train even though one is not even nearby. These phantom trains trigger a red light for the train behind it. To ensure the safety of the train and its passengers, that following train will only proceed at about 10 mph (normal speed is about 35 mph) after getting permission from the rail control center. The track circuit failure inevitably creates a backup of train traffic until dispatch can get someone out to check out what is causing the phantom train. Sometimes, it ends up being nothing more than an aluminum can on the tracks.


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