The new Orange Line trains are back in service on Monday after the MBTA determined that an improperly installed bolt had disabled a vehicle late last week.

The T announced early Monday morning that 30 of the Orange Line cars, which were manufactured in Springfield, have resumed passenger travel, four days after a breakdown at Wellington station prompted the agency to withdraw all new Orange Line and Red Line trains from the tracks.

“Preliminary results indicate that a bolt in one of the car’s eight (8) brake units was improperly installed at the manufacturing plant,” the MBTA said early Monday.

“With safety as the number one priority, MBTA vehicle maintenance personnel initiated a fleet-wide process to verify that every bolt (24 per car, 144 per train) was installed correctly. As each bolt on a six-car train passes inspection, the train is returned to passenger service. At this point in the inspection process, the problem has not been found in any other brake unit. »

An MBTA spokesperson confirmed that all new cars are assembled by Chinese company CRRC MA in Springfield.

Sixty-four new Orange Line cars and six new Red Line cars were available for passenger service, although not all were operating at the same time, before Thursday’s disruption.

The Red Line train (the six cars make up one train) had not yet returned to service as of 9 a.m. Monday morning. A spokesperson said the agency plans to bring it back online after the bolts are inspected.

The MBTA is in the midst of a multi-year contract with CRRC to replace the entire Red and Orange Line fleets with new trains, which have already encountered several problems and been withdrawn from service on several occasions.

– Chris Lisinski / SHNS