Demolition on the first of three Somerville bridges being rebuilt for the Massachusetts Bay Transportation Authority’s $2.3-billion Green Line Extension was slated to begin March 23. All three vehicular bridges are scheduled to be completed by 2020, the year before the 4.7-mile light-rail extension to Medford is supposed to open.

The long-delayed project, which broke ground in June 2018, also includes seven new stations. “This is an exciting milestone for GLX,” Steve Poftak, MBTA general manager, said in a statement.

The bridges will each be widened by approximately 34 ft to accommodate light-rail tracks, rights of way and a community path. The existing Broadway and Medford Street Bridges are only wide enough for the commuter rail tracks below them. The Broadway Bridge and its foundation will be reconstructed with a center column pier-style foundation to accommodate the new width. New east and west abutment foundations and walls, as well as a lengthened roadway surface deck, will also be constructed.

Phased work on the Washington Street Bridge includes reconstruction of abutments and structural span, improved drainage and structural improvements. Existing pier and abutment structures supporting the current Medford Street Bridge will be reconstructed.

A joint venture including Fluor Enterprises Inc., the Middlesex Corp., Herzog Contracting Corp. and Balfour Beatty Infrastructure Inc. won the $1.08-billion design-build contract in November 2017. STV Inc. is the lead design firm.

Crews recently completed the project’s first phase, which included reconstructing two railroad bridges and the demolition of MBTA-owned facilities.