Washington Street development The 1947 state act that created the
Metropolitan Transit Authority (MTA) from the
Boston Elevated Railway established four immediate projects for the new agency: extension of rapid transit to , expansion of the
Tremont Street Subway to four tracks, replacement of the existing elevated lines (
Charlestown Elevated,
Causeway Street Elevated, and
Washington Street Elevated) with subways, and an extension of the
Cambridge–Dorchester line northwest from . In 1948, the legislature authorized the city to issue $19 million in bonds (equivalent to $ in ) to construct an extension of the
Washington Street Tunnel under Shawmut Street, connecting with the existing elevated south of Dudley Square. Although none of the proposals were built immediately, it established a precedent of replacing the elevated lines. In 1972,
protests led to cancellation of the planned
Southwest Expressway. Instead, the alignment was used for
a combined corridor for intercity rail, commuter rail, and the
Orange Line – replacing the Washington Street Elevated of the latter. By 1985, the MBTA favored bus or
light rail service on Washington Street; the latter would have been a branch of the
Green Line, operated through
the 1962-abandoned southern branch. The Orange Line was rerouted in 1987; that year, the
Urban Mass Transportation Administration rejected the MBTA's funding request to create a
light rail line on the corridor. However, the MBTA closed the Elevated and instead upgraded the route 49 bus from a feeder route to a more frequent trunk route. After several more years of studies, the MBTA decided in 1996–97 to build the route as a
bus rapid transit line using
compressed natural gas (CNG) buses to avoid the visual impact of overhead wires. The Silver Line followed largely the same route between
Dudley Square and Downtown Boston as route 49; the primary change was the consolidation of stops. On July 20, 2002, new Silver Line-branded CNG buses began operation – the first low-floor buses to operate in Boston – and the 20 stops were reduced to 11. The station renaming took effect in June 2020. The November 2022 draft network plan kept the same proposal.
Waterfront development at Courthouse station in 2005. These buses were briefly used until the dual-mode buses entered service.|alt=A silver-painted trolleybus at an underground station For most of the 20th century, the
Seaport District was an industrial area occupied by
rail yards and wharves; the
South Boston Army Base and
South Boston Naval Annex were served by short bus routes that connected with the
Red Line at . After the military bases closed in 1974 and the rail yards were no longer needed, the Seaport was designated for commercial development, with accompanying need for expanded transit. The DEIR selected an underground "transitway" over alternatives including a surface light rail line, an elevated
people mover, a commuter rail shuttle, and a relocation of the Red Line. The transitway was to use
trolleybuses or
dual-mode buses, rather than the
light rail and people mover possibilities considered; it would connect with the Red Line at South Station, the
Orange Line at , and the
Green Line at . Costs could be reduced by combining its construction with the upcoming
Central Artery/Tunnel Project ("Big Dig"). Service from the transitway could be extended to serve
Logan International Airport, to form part of
a proposed circumferential transit line, and to connect with the planned Washington Street service. The South Boston Piers Transitway alignment was refined by a supplemental DEIR in 1992. Daily ridership was expected to be between 24,200 and 37,200 for the first phase, and between 34,800 and 69,800 for the full build, depending on the rate of commercial development. The environmental approval process was completed in April 1995. The discovery of a massive boulder under the Channel delayed the project by a year. The Transitway opened on December 17, 2004, with the Waterfront shuttle route between South Station and Silver Line operated by a mix of new dual-mode buses and trolleybuses borrowed from the
Harvard-based routes. Original plans called for a single South Boston route running to via D Street, Broadway, Summer Street, and E 1st Street. After concerns from residents over reduction of legal parking spaces and impacts to the "local custom" of double parking, this was split into two routes: one running to via D Street, and one to City Point via E 1st Street. Until shortly before the routes opened, the MBTA planned to distinguish them with letters like the Green Line branches, rather than the numbers that were ultimately used. On December 31, 2004, service began on routes SL2 to the
Boston Marine Industrial Park and route SL3 to City Point – the latter running via Northern Avenue, Drydock Avenue, and Summer Street. Two days later, a Sunday-only Silver Line Way-Logan Airport shuttle service called "Silver Line Connector" began operation to meet a legal commitment to begin airport service that month. As more buses entered service, the SL2 and SL3 were re-split into separate routes (except nights and weekends) on August 20, and the Farragut Road loop was cut due to noise complaints from residents. The SL3 service was never successful, as the parallel route had a more direct routing, lower fares, and better downtown connections. By 2008, the SL3 averaged less than one passenger per trip on the segment not shared with the SL2. On March 20, 2009, SL3 service was cut, with SL2 service increased in its stead. On December 22, 2019, the route was changed so that buses proceeded around the whole loop before the layover at all times. On March 13, 2022, all SL2 buses began laying over at 23 Dry Dock Avenue. The stop at 88 Black Falcon Avenue, closed since April 2019 due to pier repairs, was permanently cut at that time. An improved stop on the east side of 27 Drydock Avenue (at Black Falcon Avenue) is planned by 2025 as part of redevelopment of 88 Black Falcon Avenue. The November 2022 revised proposal kept this change, with SL2 enhanced to higher frequency service all days. SL2 service began operating every 15 minutes or better at all operating hours effective April 5, 2026.
Phase III plans The Boylston extension, as planned in 1993, would have run west from South Station under Essex Street, Avenue de Lafayette, and Avery Street. The Chinatown platform would have been under Hayward Place east of Washington Street, and the Boylston platform under the existing Green Line station, with a turnaround loop under Boylston Street and the
Central Burying Ground. The 1999 decision to combine the Waterfront and Washington Street projects as the Silver Line resulted in the addition of a southern segment, likely using the same abandoned streetcar tunnel as had been proposed a decade before. A new underground station would have been built under Tremont Street, connecting to the existing (NEMC) station, with a portal to Washington Street just north of Oak Street. In 2003, the preferred portal location was moved slightly north to avoid the need to demolish the YMCA building, and the NEMC station was removed. In February 2006, State Transportation Secretary John Cogliano proposed a $94 million plan that would eliminate most of the tunneling and cost of the original proposal while still connecting the two phases of the service. The plan was popular with Bay Village residents who had been worried about the full-length tunnel, but attracted criticism because it would not substantially speed travel times to downtown. This "Charles Street Modified" alignment remained the preferred alternative for the remainder of the project. Contra-flow dedicated bus lanes, already in place on Washington Street, were to be extended onto Marginal Road and Herald Street to allow buses to reach the portal from the surface section. On December 12, 2006, the FTA approved the project to re-enter its funding process. However, the estimated price of the tunnel plan, dubbed the "Little Dig", had risen to $2.1 billion by May 2009. The FTA assigned it a
Medium Low overall rating, making it ineligible to move into the final design phase for federal New Starts funding. The Boston Region Metropolitan Planning Organization removed Phase III from the list of recommended projects in its long range plan because of funding limitations. Concluding that it could not successfully compete for more than one New Starts grant, the
Massachusetts Department of Transportation (MassDOT) informed the FTA that it was no longer seeking New Starts money for Phase III. All New Starts funds available would instead be directed to the legally mandated
Green Line Extension project. In a July 2010 report, the MBTA declared that Phase III was on indefinite hold and no further funds would be spent on the project. A partial solution that did not require a new tunnel opened on October 13, 2009, after expedited construction using
federal stimulus money. The new route, SL4, covered much of the same alignment as the proposed Phase III, with a dedicated bus lane on Essex Street and a South Station stop west of Atlantic Avenue. The existing Downtown Crossing-Dudley route was renamed SL5 at that time.
Extension to Chelsea The
Urban Ring Project was a planned circumferential surface BRT line. It would have shared the Silver Line berths at Dudley Square, and have had a surface stop outside World Trade Center station, but otherwise would have been separate from the Silver Line. The project was cancelled in 2010 due to high cost, but planning for several smaller sections continued. The
Chelsea–South Boston section was given high priority because Chelsea was densely populated yet underserved by transit. In 2013, MassDOT began public planning of the
Silver Line Gateway project. In addition to serving Chelsea, the proposed line would provide
Blue Line riders a direct connection to the Seaport and South Station. Three possible alignments were studied. All used the Transitway and the Ted Williams Tunnel to reach station, then the 2012-opened Coughlin Bypass Road to the
Chelsea Street Bridge. The first alignment was to run entirely on a new busway on the Grand Junction right-of-way in Chelsea, with stops at , , at the Chelsea commuter rail station, and
Mystic Mall. The second alignment option would have followed the Grand Junction to just short of the commuter rail station, then diverge onto surface roads to Bellingham Square, while the third alignment would have run entirely on surface streets, serving two stops on Central Avenue and four stops along a loop serving Chelsea station and the MGH Chelsea healthcare center. In September 2013, the MBTA indicated that it would pursue the first alternative, with an estimated daily ridership of 8,700, despite potential issues with bridge clearances and rebuilding the commuter rail station. On October 30, 2013, MassDOT announced $82.5 million in state funding for the new Silver Line route to Chelsea, with completion expected in 2015. The announcement also included the relocation of the commuter rail station to (Mystic Mall) and a $3 million, multi-use path from Eastern Avenue to Washington Street. The Environmental Impact Report was issued in March 2014. MassDOT awarded a $33.8 million construction contract for the first phase of the project on September 17, 2014. That phase included the -long busway, the four Silver Line stations, replacement of the Washington Street bridge, and the
Chelsea Greenway. Silver Line service to Chelsea (route SL3) began on April 21, 2018. By October, daily ridership reached 6,200. , the MBTA and the City of Boston planned to reconfigure
Day Square in East Boston in late 2024. The project would include bus lanes on Chelsea Street and a bus-only street through Day Square, with a Day Square station for use by the SL3 and local bus routes.
Sullivan Square extension In March 2019, state and local officials indicated plans to extend service from on two routes to
Kendall Square and to
North Station, both via
Sullivan Square. The routes would primarily use dedicated busways and bus lanes, although it was not decided whether they would be under the Silver Line brand. In January 2021, the MBTA and MassDOT began work on the Silver Line Extension (SLX), which would look into route alternatives from Chelsea into Everett, Somerville, Cambridge, and Boston. Six potential corridors were identified by September 2021: to , to Sullivan, to , to downtown Boston via Sullivan, to Kendall Square via Sullivan, and to Kendall Square via Wellington. Some corridors could be operated as new routes independent from the SL3, with Chelsea or Airport as potential termini rather than South Station. Alternatives were presented in September 2022. Three potential SL3 extensions would run to Malden Center, Wellington, or Sullivan – all via Everett Square, with varying amounts of dedicated bus lanes on the western portion. Four potential SL6 routes were identified: Everett (Glendale Square)–Kendall via
McGrath Highway, Everett–Kendall via the Gilmore Bridge, Everett–Haymarket via
Rutherford Avenue, and Eastern Avenue–Kendall via Everett Square. All of the potential SL6 routes would operate primarily in dedicated lanes. A SL3 extension to Sullivan was announced as the preferred alternative in March 2024; the study recommended that SL6 planning be deferred pending related planning efforts for Sullivan Square, Rutherford Avenue, the Gilmore Bridge, and Kendall Square, as well as implementation of the planned bus network redesign. In June 2024, the MBTA was awarded a $22.4 million federal grant for reconstruction of the Lower Broadway segment for existing bus routes and eventual Silver Line service. Conceptual plans for the Sullivan Square–Sweetser Circle section released in March 2026 called for a bidirectional busway on the north side of Broadway/Alford Street with three stops located at Bartlett Street, Thorndike Street, and Dexter Street. Northbound buses would operate in mixed traffic from Beacham Street to Sweetser Circle.
Other proposed corridors near the former
Pleasant Street portal. This tunnel was briefly considered for use in the Phase III tunnel and is the likely connecting route for a proposed conversion of the Washington Street section of the Silver Line to a branch of the Green Line.|alt=View through a two-track railway tunnel Several other bus rapid transit and express bus projects have been proposed in Boston, many under the Silver Line banner. The first two phases of the 2010-cancelled
Urban Ring Project were to be BRT, with light or heavy rail for the final phase. The Urban Ring was considered a separate project, although it would have shared the SL1 route between Silver Line Way and Logan Airport. However, the proposal was withdrawn in 2010 due to local opposition, both to the design of the route and because the plan had been made without consulting local officials. Subsequent planning for bus improvements on Blue Hill Avenue has occurred without Silver Line branding. Several other corridors were considered in the 2003 PMT. These included a Dudley–Ashmont route replacing the
route 23 bus (also revived in 2006 in Phase III planning, but not during the 28X proposal), as well as a new BRT tunnel to
Kenmore with surface branches to the
Longwood Medical Area via
Brookline Avenue and
Allston via
Commonwealth Avenue, the
Mass Pike, and Cambridge Street. The 2003 PMT included the possibility of converting the Washington Street section of the Silver Line to light rail (as had originally been promised) using the abandoned southern section of the
Tremont Street Subway. The project was estimated to cost $374 million; ridership was estimated to be 34,000 daily riders almost entirely diverted from the Silver Line service. The project was given low priority, with the Phase III tunnel recommended instead. ==Service==