November 17, 2015

Mysteries of New York: Signaling systems in the subway

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From The Atlantic:
Subway

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This complex—of towers, signals, switches, and track sections—is responsible for a disproportionate share of the costs and foibles in the operation and maintenance of New York’s subway system.
The equipment is old and breaks all the time. In fact it’s so old that the MTA can no longer buy replacement parts from the manufacturer; it has to refurbish them itself. Some of the controls for the interlockings are originals from the 30s. Much of the wiring is still insulated with cloth, instead of rubber; ten years ago the entire Chambers Street interlocking caught fire. Salt water from Hurricane Sandy did damage to trackside switches and signals that is still being repaired. …
The equipment is old and breaks all the time. In fact it’s so old that the MTA can no longer buy replacement parts from the manufacturer; it has to refurbish them itself. Some of the controls for the interlockings are originals from the 30s. Much of the wiring is still insulated with cloth, instead of rubber; ten years ago the entire Chambers Street interlocking caught fire. Salt water from Hurricane Sandy did damage to trackside switches and signals that is still being repaired. …
You want fewer delays? You want realtime countdown clocks?
CBTC is the answer. …
CBTC does away with the “fixed-block” signaling system, where track is broken into sections that report whether they’re occupied. Instead, each train is equipped with a radio and onboard computer that identifies its precise location, and coordinates that information in real time with a central control center and other trains to decide exactly how fast it can safely go. Trains therefore run with a moving window around them, which constantly shifts depending on their own speed, size, track conditions, and traffic. …
New York decided to run CBTC on top of a reduced form of the old fixed-block signaling system, requiring that both be expensively maintained, despite evidence from other cities that no backup was necessary. (In Vancouver, the SkyTrain has had no CBTC-related accidents in more than 26 years.) And the fact that the two systems had to work together—requiring the supplier to study the old signals in depth—became a major source of delays. …
The MTA thought that they could buy a software solution more or less off the shelf, when in fact the city’s vast signaling system demanded careful dissection and reams of custom code. But the two sides didn’t work together. The MTA thought the contractor should have the technical expertise to figure it out on their own. They didn’t. The contractor’s signal engineer gave their software developers a one-size-fits-all description of New York’s interlockings, and the software they wrote on the basis of that description—lacking, as it did, essential details about each interlocking—didn’t work.
Complete story here.

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