BOSTON (WHDH) - The new Orange Line trains are back in service after nearly four days of investigations and inspections, the MBTA announced.

The cars were taken out of service last week after one of the new trains experienced an issues in one of its braking units.

“Out of an abundance of caution, the MBTA removed all new cars from service while engineers and technicians worked to determine what caused the problem,” the MBTA said in a statement.

MBTA official said that one bolt in one of the car’s eight braking units had not been properly installed at the manufacturing plant. The MBTA then started a fleet-wide process to check each bolt. There are 144 bolts on each train. While the process is still underway, no other issues have been detected, and the inspected cars are being returned to service.

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