THE MBTA said on Thursday that it needs to extend the shutdown of service on a portion of the Blue Line a second time because a tool cart being used to lay 1,800 feet of new track derailed near Airport station earlier this week.

The T said the shutdown between Airport and Bowdoin stations is being extended through Tuesday with service scheduled to resume on Wednesday.

The transit authority, which is struggling to deal with a wide variety of safety issues and facing increased oversight from the Federal Transit Administration, has been tight-lipped about problems with the Blue Line construction work.

On Sunday, regular service was scheduled to resume on the Blue Line after a 14-day closure, but the agency announced it was extending the shutdown through Friday “to finish scheduled work and complete additional work.”

Only when pressed about reports of problems with the repair work did the agency acknowledge the delay was needed for safety reasons because of the configuration of the track.

“With safety always as the top priority, it was determined that additional time was needed to ensure the new track installed in these complicated areas is properly in place and safe for trains to travel over when Blue Line service resumes,” the T said on Sunday.

On Thursday, the T announced the shutdown was being extended into next week because a construction tool cart had derailed. “There were no injuries,” the T said in a statement. “The process to re-rail the tool cart earlier this week and make other repairs while continuing to finish scheduled work means additional time is needed to safely complete the project.”

The main contractor on the project is Railworks Track Services LLC. There were reports the contractor had been ordered to stand down but a T spokeswoman could not immediately confirm that. She could also not confirm whether the contractor would be responsible for the costs of the extended shutdown.

The MBTA said details regarding alternate shuttle bus service and ferry schedules are available at mbta.com/harbortunnel.