The MBTA will shut down the Green Line extension branch between Lechmere and Union Square from Sept. 18 to Oct. 13.

The Squires Bridge over the MBTA tracks in Somerville — which carries 40,000 vehicles a day — needs repairs. In order to do the job, the T has to suspend service on those tracks.

The Squires Bridge repairs were supposed to be completed earlier this summer, but local officials balked at the lack of adequate warning. So, the T rescheduled the work and shortened the project timeline from 42 to 25 days.

Highway Administrator Jonathan Gulliver says the new schedule to repair the bridge will minimize the impacts to transit service.

“We know this will come as an inconvenience to Green Line riders and we have worked to adjust our construction schedule to minimize as much as possible while balancing the need to complete necessary infrastructure work,” Gulliver said.

Service on the longer Green Line extension branch to Medford will not be affected by the bridge work. Neither will service on the nearby Fitchburg Commuter Rail line.

Unlike other subway closures, MassDOT will not be providing shuttle buses as an alternative means of transportation while the Union Square branch is out of service. Riders are encouraged to use existing bus routes 86, 87, 91 or CT2.