Four days after the T announced it would again extend the shutdown of Blue Line subway service between Maverick and Bowdoin because a “construction tool cart derailed,” MBTA General Manager Steve Poftak told reporters there have in fact been three such incidents during the maintenance effort.

A project to replace 1,800 feet of Blue Line track, upgrade lighting, repair signals and perform other maintenance work was originally supposed to wrap up on May 8, but a series of incidents have pushed the end date to Tuesday, May 17, forcing passengers who need to access several stops to use replacement shuttle buses for longer than expected.

Poftak said each of the three derailments involved a “tool cart” and that two of them occurred as the construction vehicle was traveling over a section of rail under construction.

“Derailments are never a good thing, but just to clarify, these were not Blue Line vehicles, not revenue vehicles, they were tool carts being used on the construction,” Poftak said. “We fully reported in full compliance with the safety regulations that we work under. These were reported to the [Federal Transit Administration] and [Department of Public Utilities] in a timely manner, and obviously, the site was frozen and full investigations undertaken each time.”

Asked if any of the incidents caused track damage, Poftak said he was unsure and that work crews are in the process of completing the project.

“We have set aside plenty of time for our internal teams to do a full inspection not only of the work and of the track – I have it up on my board in my office,” Poftak said. “I can assure the public that we will inspect every inch of that project before we resume revenue service.”

A second Blue Line shutdown, originally planned to take trains between Wonderland and Orient Heights offline from May 12 to May 29, is on hold as a result of the delays that have plagued the project on the other end of the subway line.