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The Massachusetts Bay Transportation Authority put one of its two new Orange Line trains back on track Tuesday, saying it had fixed a mechanical problem that arose shortly after the highly awaited vehicles entered passenger service last year.

The two six-car train sets were pulled from service in mid-November, after inspectors grew wary of an “uncommon noise” later attributed to a pad between the cars’ main structural components. The T has in recent weeks been testing a fix for the issue, but declined to say when the trains would return to service.

For passengers who have suffered through poor service and on filthy vehicles for years, the Orange Line trains represent a tangible sign of hope in the T’s highly publicized but yet-unrealized quest for improvement.

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The first of the trains was introduced to service with much fanfare last year by Governor Charlie Baker, celebrating them as a realization of those dreams. But on Tuesday, MBTA General Manager Steve Poftak cautioned riders that the trains may face more hiccups as workers continue to break them in.

“The MBTA will continue to closely monitor the new equipment and require vehicle engineering staff to be aboard for each trip to monitor performance,” Poftak said in a statement. "By subjecting new trains to this high level of scrutiny, the MBTA can proactively identify any potential issues and take any corrective actions early. ... Going forward, train sets may be taken out of service from time to time as potential issues are detected that may require further analysis.”

Railroad experts have told the Globe that it is common for new trains to face issues as they enter service and are faced with their first passenger loads. And Poftak on Tuesday said it is important for the MBTA to be especially cautious with the first cars in the new fleet, because the T is about to receive hundreds of new cars on the Orange and Red Line cars. Catching problems now means the solutions can be incorporated with the production of the rest of the fleet, he said.

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This marked the second problem for the new subway cars. The T previously pulled them from service in the fall to address an issue that caused a door to open mid-trip.

That incident removed the cars out of service for less than two weeks, compared to a month and a half with the latest incident.

This time, a 5-by-13-inch pad that goes between the upper and lower parts of the subway cars — the truck, or undercarriage, which includes the wheels, and the body, which carries passengers — was “wearing irregularly,” according to the MBTA. The pad helps manage the interactions between those major structural components as trains curve through tunnels, for example.

MBTA spokesman Joe Pesaturo said the “corrective action” was to provide a “smoother surface finish” for the wear pad.

So far, only one of the two new car sets is back on the tracks. Pesaturo said the second will come later this month after it is fully equipped with the replacement pads. The cars are under warranty as they are introduced to service, so the fixes do not cost the T any money, Pesaturo said.

They are being partially built in China and assembled at a massive Springfield factory by the Chinese-owned CRRC MA. The company has contracts for about $1 billion to build 152 new Orange and 252 Red Line cars, fully replacing each fleet.

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MBTA officials say they expect the first Red Line train to enter passenger service this spring, though the T has not yet finished construction on a small South Boston facility that will be used for systems testing. Pesaturo said the shed-like facility should be completed in the next several weeks; the agency has completed work on a nearby test track.

The T is also in the process of introducing two-dozen new Green Line cars, which were purchased as part of the light-rail line’s expansion through Somerville. So far, nine have been approved to serve passengers, more than a year after the first one hit the tracks.



John R. Ellement can be reached at john.ellement@globe.com. Follow him on Twitter @JREbosglobe. Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter at @adamtvaccaro.