The electric substation on the Green Line extension branch to Union Square. (Photo courtesy of MBTA.)

MBTA GENERAL Manager Steve Poftak said the Green Line extension to Union Square in Somerville will officially open for business on March 21, while the branch to Medford is being pushed back from May to sometime in late summer.

At a meeting of the MBTA board on Thursday, Poftak said the subway line opening would be the T’s first since 1987.

“This is the T delivering on its commitment to get this open in March,” he said. “It’s a symbol we can do big things and get things done.”

With the opening, T riders will be able to ride from Union Square in Somerville to a new Lechmere Station and then on to a reopened Science Park station before joining the existing Green Line network.

The Green Line work has been delayed several times toward the end of the project, but it is expected to come in under the $2.3 billion budget with enough money left over to reimburse Somerville and Cambridge for the $75 million they put up as part of the original financing package.

Bruce Mohl oversees the production of content and edits reports, along with carrying out his own reporting with a particular focus on transportation, energy, and climate issues. He previously worked...