THE MBTA is extending the shutdown of a portion of the Blue Line for another five days to make sure the track being installed is safe.

The T announced on Sunday that it was extending the original 14-day shutdown until Friday “to complete additional work and finish scheduled work.” On Monday, asked about reports that the project had encountered difficulties, a T spokeswoman acknowledged in an email that the work was proving challenging.

“The location of this track work along some areas of the Blue Line is a complicated area that involves both sloping of the track (meaning the track rises and descends) as well as a curve,” said T spokeswoman Lisa Battiston. “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.”

Battiston did not address any specific obstacles or cite any specific problems. She also did not say whether the contractor on the $5 million project was being held responsible financially for the delay.

CommonWealth was told an initial portion of the track work needed to be redone because a test of the rails failed.

The project involves the replacement of 1,800 feet of track and a wide assortment of other tunnel improvements while that work is being done. During the construction, shuttle buses will run from Government Center to Maverick where Blue Line service will resume. There also will be alternate ferry service from Lewis Mall near Maverick to Long Wharf. Details can be found here.

The five-day extension applies to a shutdown of the Blue Line between Airport and Bowdoin stations. Because of that extension, a shutdown of the Blue Line between Wonderland and Orient Heights from May 12 to May 29 to work on the Suffolk Downs pedestrian bridge is being postponed.

In announcing the initial closure of the Blue Line, MBTA General Manager Steve Poftak acknowledged the inconvenience but said the tradeoff is much more rapid completion of the work. “We know these diversions can be an inconvenience, but these service suspensions allow us to expedite critical tunnel and infrastructure work. I want to thank our Blue Line riders for their patience while we accelerate these important projects,” Poftak said.