Metro

MBTA says changes are coming after its signal system again strands riders

Craig F. Walker/Globe Staff/File
Travelers waited for a Red Line train at the Park Street Station in Boston.

The severe delays that struck the Red Line Tuesday morning and stranded commuters for several hours exposed the subway system’s aging signal system, an Achilles heel that the MBTA will spend hundreds of millions of dollars to fix over the next six years.

A lone sensor on a rail switch near Kendall Square failed around 6:20 a.m., causing Red Line trains in the immediate vicinity to freeze, triggering a cascade of delays up and down the line that took 3½ hours to clear.

Another signal failure, at Alewife, caused delays during the evening commute, too.

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The snafus came on the first day after the long Labor Day holiday weekend, with ridership returning to more normal levels after the August vacation lull.

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A top Massachusetts Bay Transportation Authority official estimated that between a quarter and a third of all subway delays are the result of signal issues. There were at least 10 in the last two weeks alone, with the longest, on Aug. 23, taking more than four hours to resolve.

“They’re really the bane of every subway rider’s existence. If you ask them what’s the message they dread the most, it’s signal problems. If you’re sitting on the train and know there’s a signal problem, you know you’re going to be delayed,” said Rafael Mares, a transit advocate with the Conservation Law Foundation. “And unlike a medical problem or police activity, it’s at least theoretically controllable for the T.”

Regular subway riders have become all too familiar with the MBTA’s dreaded announcements and alerts about signal troubles. “Oh great. Signal problems before the holiday weekend, signal problems after,” one rider posted on Twitter Tuesday morning.

The T plans to spend $352 million to improve the signal systems on the Red and Orange lines before new subway cars for each line arrive by 2023.

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In the meantime, maintaining a system that dates to the 1970s is a huge task. The signal system is essentially a long web of lights, sensors, circuits, and wiring that’s used to manage traffic along the rails and prevent trains from running into one another. Because signals serve such a key safety function, subway systems are very sensitive to any type of problem with them.

“It’s the only way you can run multiple trains on the same track safely and efficiently,” said Augustine Ubaldi, a railroad expert with the Pennsylvania firm Robson Forensic, which provides expert witnesses in litigation. “It’s vital equipment. It’s a safety priority. If my signal system doesn’t give me the proper indications, my train grinds to a halt or it moves at restricted speeds to ensure safety.”

That is essentially what happened Tuesday morning on the Red Line. The sensor in question, one of several inputs to the signal system, had a “poor connection” and needed to be replaced, MBTA Deputy General Manager Jeff Gonneville said. The sensor was unable to determine which direction trains would travel, so as a safety measure the signal system prevented trains from traveling as they normally would.

The malfunction was just one of an untold number of reasons the signal system can fail, Gonneville said.

“We’ve got over 10,000 signal relays, and we have over 100 switches on the Red Line, as well,” Gonneville said, and each plays a role in triggering signals. “If you stretch it across all lines, there are thousands and thousands of points of failure where it could cause a disruption or delay.”

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Signal problems are so pervasive, in part, because they can be caused by problems with any number of pieces of equipment. Some are unpredictable. In June, for example, a fire on the Green Line affected a wire that turned a signal off, resulting in significant rush-hour delays.

Some issues, like exposed circuits, are easier to diagnose and can be dealt with in half an hour. Other problems, like Tuesday’s, are more difficult to identify and fix. The longer it takes to find the problem, the greater the delays will be along the subway line.

On Tuesday, for example, train operators near Kendall Square needed to manually bypass the broken signal to keep moving. But to bypass the signal, they must undergo a complex process to get permission. Even then, trains can go a maximum of 25 miles an hour, and just one train can operate on a stretch of track at a time.

That meant that on Tuesday morning, only one reduced-speed train was traveling in each direction between the Charles/MGH and Central Square stations at a time.

“It limits the amount of trains running through that area and causes congestion across the line,” Gonneville said.

The T could have shortened the reduced-speed zone, but did not “out of an abundance of caution,” he said, because the MBTA’s operations center has limited control over trains when they are in bypass mode. The MBTA uses a “fixed block” signal system, in which trains are unable to access stretches of track until the train in front has cleared that segment.

Some transit advocates have called for the T to install a more modern but more expensive system that lets trains communicate their locations to each other and move accordingly. Such a system is in the works in New York, though its implementation has been long delayed. Last year, MBTA officials said that type of communications-based system would not significantly improve service on the Red Line and was probably not worth the expense.

The T is planning to improve the fixed block signals on the Red and Orange lines, replacing an analog system with a digital one and upgrading old equipment. The computerized system will make it easier to identify and more quickly fix problems, Gonneville said. Contracts for most of that work are expected to be put out to bid in the coming months.

Adam Vaccaro can be reached at adam.vaccaro@globe.com.