Rajee Alexander, PE, CSEP

Senior Communications Engineer

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Jul 1, 2020

Technological advances continue to push Communications Based Train Control systems to new levels, as increasing numbers of rail agencies across the US implement CBTC to improve both safety and efficiency.

Communications Based Train Control (CBTC) is an automatic signaling system that has been used in heavy rail, light rail and automated people mover industries for more than three decades. Relying on telecommunications between the train and track, CBTC can precisely determine a train’s location and its distance from other trains, and can warn operators of problems before they affect service or safety. Knowing precisely where trains are helps shorten the time and distance, or headway, between trains. Reducing headway between trains allows rail agencies to put more, longer trains in service and run them more efficiently and reliably. The efficiency achieved, in turn, keeps customers happy and better positions rail agencies to meet growing passenger demands.

“CBTC can precisely determine a train’s location and its distance from other trains, and can warn operators of problems before they affect service or safety.”

CBTC replaces legacy signaling systems that have been used in some locations since the 1930s, or earlier. These manually operated “fixed-block” systems employ electric track circuits, spaced at specific, 1,000-foot intervals or “blocks,” to help determine a train’s location. Ideally, the circuits and signals keep one train from entering a block that another is occupying. Electric signals installed alongside the track, much like traffic lights, let train operators know when the block ahead is free for travel. While the fixed-block system has been used for decades, it cannot detect speed or other nuances, and there have been problems, especially as these legacy systems age well past their intended obsolescence.

Fixed-block systems also don’t provide the precise location, speed and performance information that CBTC does. With CBTC, a train is constantly receiving information on its headway from other trains, and can adjust its speed and distance accordingly.

Now, most CBTC systems operate using radio communication or indictive loops, which enable signals to be passed to and from wayside transponders and on-board radio systems to monitor the train’s position, speed, distance from other trains, and required braking distance. CBTC systems have been installed and are operating with success on several rail lines in the US, including New York City Transit’s Manhattan-to-Brooklyn Canarsie Line and its Manhattan-to-Queens Flushing Line; PATH in Jersey City, New Jersey; and SEPTA’s Green Line in Philadelphia. In addition, CBTC upgrades or installations are underway in San Francisco, Baltimore, suburban Philadelphia and Queens, New York City.

Since completing CBTC installation and signaling upgrades on its Flushing “7” Line in 2017, New York City Transit has been able to boost on-time performance to 92 percent—a more than 20-percent increase in reliability. Similar increases in reliability have been reported by other agencies.


The use of radio-based CBTC, however, could soon be eclipsed by the next technological evolution: ultra-wideband. Although some rail agencies have supplemented their existing CBTC systems with GPS technology to detect trains’ exact locations, GPS doesn’t always work well in tunnels, underground, in cavernous stations, or others areas that are impervious to satellite transmissions. Ultra-wideband doesn’t pose those limitations, and affords continuous communication and data transmission, as well as location identification to within a few centimeters. Ultra-wideband also provides information on track conditions, and can let operators know whether an object—or a worker—is on or dangerously near the tracks.

Like CBTC, ultra-wideband uses transmitters installed on trains send data on speed, location and performance to small “receiver” units placed at distinct intervals along the wayside, providing both the operator and central control with the real-time information they need to make quick decisions. Unlike CBTC, wireless ultra-wideband wayside units can be installed quickly and easily, avoiding costly, schedule-sabotaging track outages. Once the system infrastructure is in place, onboard wireless transmitters and wayside units can be installed in weeks or months, rather than years, as is the case with “wired,” radio-based CBTC systems. The resultant cost and time savings then, ideally, could be spent on other improvement projects that need attention.

Ultra-wideband determines train location “within a few centimeters.”


The wireless technology also is more easily integrated with both CBTC systems and older, fixed-block systems—a hurdle the CBTC alone has long struggled to surmount.

Already, ultra-wideband is being prescribed for new, “greenfield” rail lines that are being built from scratch in growing regions across the country. However, the ease of installation of the technology could prove it ideal for older, urban “brownfield” rail lines as well.

The New York Metropolitan Transportation Authority and its New York City Transit Authority recently completed a nine-month test of ultra-wideband on its Canarsie “L” and Flushing “7” subway lines, and plan to roll out the technology to other lines as well. Integration with the lines’ existing CBTC systems was key to the success of the pilot program.

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“… wireless ultra-wideband wayside units can be installed quickly and easily, avoiding costly, schedule-sabotaging track outages….”


“Ultra-wideband wireless technology brings the promise of fewer and shorter delays, and faster and cheaper installation of modern CBTC signaling, by eliminating much of the equipment traditionally fitted under trains and on tracks,” said former NYCTA signaling lead Pete Tomlin. “This is a game-changer for our customers.”

While the new technology’s promise it still being studied, it has a bright future in helping both new and legacy rail lines achieve greater levels of efficiency, reliability and customer service—which is good news for both rail agencies and passengers alike.