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Once again the new Orange Line subway cars have been taken out of service, and once again the problem is in the underbody of the vehicles where the chassis meets the carriage.

The MBTA announced Tuesday morning the two six-car trains were pulled from service, and officials said they expect them back online later this week. Spokeswoman Lisa Battiston said the “exact cause of the issue is currently under investigation."

On Twitter, the T said inspectors had found a problem with the trains’ bolsters, leading to their removal from service.

Bolsters are a component that helps connect the cars’ bodies, where riders sit and stand, with the truck, an undercarriage that includes the wheels. The purpose is to keep the body in relatively stable position while the truck changes position as it curves on tracks.

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Bolster problems are already familiar on the new Orange Line trains, cited as the cause of a problem that surfaced last fall and took the new trains out of service for about two months.

At the time, T officials disclosed that two separate pads — one on the bolster, one on the truck — were grating against each other improperly, signifying a potential problem in the interaction between the truck and the body. The T returned the trains to service shortly after replacing the bolsters.

This diagram shows the part of the subway car's undercarriage where the bolsters are located. It was released by T officials in January to demonstrate where a previous problem with the new cars was located.
This diagram shows the part of the subway car's undercarriage where the bolsters are located. It was released by T officials in January to demonstrate where a previous problem with the new cars was located.MBTA

“The purpose that they provide is they essentially help regulate that rotation of the train,” MBTA deputy general manager Jeff Gonneville said at an agency board meeting on Jan. 13. “The forces between the sandwiching of those two pads essentially limits the rotation of the truck. You don’t want it spinning too loosely, and obviously you do not want it too tight.”

Battiston declined to say whether the latest bolster issue was the same problem or a new one. CRRC, the Chinese company that is assembling more than 400 new Orange and Red Line cars at a Springfield factory, also did not respond to a request for comment.

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Their removal for even a few days is another setback for the project that, by the end of 2022, is supposed to replace and expand an aging set of Orange Line cars that date back 40 years. Meanwhile, delivery of a third train is already behind schedule.

The first new six-car trains were introduced into service last summer, months later than expected, and have already been taken away from riders twice. They were taken out of service in September to fix a door component. MBTA officials reported that the door lock mechanisms were replaced at the manufacturers’ expense.

Since the 12 cars returned to service in January, the T has emphasized they may again come offline from time to time to address defects as they arise. Even before the trains first entered service, the T and CRRC had convened a “failure review board,” an industry term for a working group that addresses problems that surface during testing.

Officials have said it is better to be extremely cautious with a few cars as they enter service so that problems can be fixed proactively before the rest of the fleet is built, rather than requiring far more trains to come out of service later; New York City, for example, recently pulled about 300 new trains from service because of a problem with the doors.

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And railroad experts have told the Globe that it is common for manufacturers to run into hiccups as they enter service.

“It’s embarrassing to only have a fraction of the fleet delivered and having repetitive failures,” said Bradley Clarke, a local rail car expert with the Boston Street Railway Association. “But as a practical matter, it probably is typical.”

Still, Clarke said, it doesn’t have to be this way. He pointed to the introduction of new Blue Line cars built by industrial giant Siemens more than a decade ago. These, too, experienced problems at the outset, but the setbacks and delays came before the cars went into service. Once on the Blue Line, he said, there was not much looking back.

“Those Blue Line cars have been the most trouble-free cars ever introduced on the Boston system,” Clarke said. “They had very, very few problems. Yes, there were tiny bugs and so on ... but you weren’t taking the entire fleet out of service.”

Clarke said the bolster problems on the Orange Line are “too trivial” to not be fixed soon, but he wondered whether the problems may fray the relationship between the MBTA and CRRC, which recently expressed interest in building Green Line cars for the T as well.

CRRC also recently missed a planned delivery of new cars to Boston, delaying the introduction of a third train set by about a month. The T said CRRC is working to improve productivity at the plant.

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The company said it was adding 20 employees to the factory soon, although spokeswoman Lydia Rivera also said understaffing was not the cause of the delay. And Paul Gour, business agent for the International Brotherhood of Electrical Workers Local 7, which represents some plant employees, said he was unaware of any productivity issues.

“They keep hiring and everything seems to be moving along,” he said.

The T announced on Twitter the trains would be sidelined Tuesday. Previously the agency has been criticized by transit activists as not being proactive in communicating problems, especially about the new subway cars; only when pressed by reporters had officials acknowledged the two previous times they were taken out of service.


Emily Sweeney can be reached at emily.sweeney@globe.com. Follow her on Twitter @emilysweeney.