Much of the system is missing BRT Standard features such as enforced dedicated lanes, off-vehicle fare collection, sheltered stations, and transit signal priority.

A single New Flyer diesel hybrid bus with extended battery range was obtained as an option on a separate order; it entered testing in September 2018 and revenue service in December.

[13] In November 2020, the MBTA exercised a contract option for 45 additional 60-foot hybrid buses with extended battery range (similar to test bus #1294) to replace the dual-mode Silver Line fleet.

[5]: 27  This was inherited from the previous route 49 bus, which had free transfers (with the flat token fare) to and from the Orange Line only at New England Medical Center.

[7]: 3.1  By 1990, the MBTA expected service to begin in 1993, with an underground connection to Boylston station and the proposed South Boston Piers tunnel in a future phase.

[35] 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.

[3] 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.

[58][3] In January 2021, Massport approved a ground lease agreement for air rights development over Silver Line Way station, which will include improvements to the stop.

[49]: 2.5–6  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.

[61] 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.

[49]: 2.15–22  Further changes in 2003–04 were caused by the need to add a second platform at Boylston due to high expected ridership, and to reduce impacts to the historic Boston Common from the loop.

[49]: 3.33  In August 2005, the MBTA put the Phase III project "on hold" in order to build community consensus on a locally preferred routing.

The plan also included expansion of surface Silver Line service, with a new branch running from Copley Square into the Essex Street portal to provide a one-seat ride from the Back Bay area.

[4]: 27  By mid-2008, environmental review and preliminary engineering were expected to be completed by the end of the year, with federal funding sought in 2010 and construction lasting from 2011 to a 2016 opening.

[70] 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 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.

[75] MassDOT had also purchased the Grand Junction Branch – including abandoned sections in Chelsea and East Boston – from CSX Transportation in 2010, meaning that a potential right-of-way (which the Urban Ring had proposed to use) was available.

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.

[77] 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.

The announcement also included the relocation of the commuter rail station to Chelsea (Mystic Mall) and a $3 million, 3⁄4-mile (1.2 km) multi-use path from Eastern Avenue to Washington Street.

[88] 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.

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.

[92][93] 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.

One, a BRT express overlay for the route 28 bus (which runs between Ruggles station and Mattapan via Nubian), was revived in 2006 as part of the Phase III plans.

[99] In 2012, the Roxbury–Dorchester–Mattapan Transit Needs Study recommended the conversion to light rail as a long-term project, with the additional possibility of extending the line down Blue Hill Avenue to Mattapan along the route 28 bus corridor.

[70] Although transit ridership in the Seaport doubled after the introduction of the Silver Line, that growth is due partially to commercial development rather than the service quality.

[4]: 12  A significant portion of delays on the surface sections are caused by long dwell times, as passengers only board through the front door to pay at the single farebox.

[110]: 136  Additional enforced downtown bus lanes, plus traffic restrictions on Temple Place and new loading zones in Chinatown, were added by the city in 2020.

However, the BTD's modifications did not modify the unusually-long 100-second cycle length nor actively prioritize buses, leading to criticism from transportation planners that the solution was inadequate.

[134] The SL3 route is subject to frequent delays due to the opening of the Chelsea Street Bridge – as many as ten times per day – for ships serving the upstream oil terminals.