BOSTON (SHNS) – New Orange Line trains are rolling back into service Monday after the MBTA determined an improperly installed bolt disabled one vehicle late last week.

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

“Preliminary findings indicate that one bolt in one of the car’s eight (8) braking units had not been properly installed at the manufacturing plant,” the MBTA said early Monday. “With safety as the number one priority, MBTA vehicle maintenance personnel started a fleet-wide process to verify that each bolt (24 per car, 144 per train) was properly installed. As each bolt on a six-car train passes inspection, the train is being returned to passenger service. To this point in the inspection process, the issue has not been found in any other braking units.” An MBTA spokesperson confirmed that all of the new cars are assembled by Chinese-owned firm CRRC MA in Springfield.

Sixty-four new Orange Line cars and six new Red Line cars had been available for passenger service, though not all were running 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 planned 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 faced several issues and been removed from service on multiple occasions.