The Silver Line is a bus route system in Boston and Chelsea, Massachusetts, operated by the Massachusetts Bay Transportation Authority (MBTA). It is operated as part of the MBTA bus system, but branded as bus rapid transit (BRT) as part of the MBTA subway system. Six routes are operated as part of two disconnected corridors. As of 2026, weekday ridership on the Silver Line was 37,905.
The four Waterfront routes operate out of an underground terminal at South Station and run through the South Boston Piers Transitway – a dedicated bus tunnel through the Seaport District with stations at Courthouse and World Trade Center. At Silver Line Way, they fan out on the surface: the SL1 to Logan International Airport, the SL2 to Dry Dock Avenue, and the SL3 to Chelsea via East Boston. An additional short turn route, SLW, runs only at peak hours between South Station and Silver Line Way. The Waterfront routes use mostly articulated diesel hybrid buses with extended battery range. Two routes operate on Washington Street between Nubian station (at Nubian Square in Roxbury) and Downtown Boston. The SL5 terminates at Downtown Crossing and the SL4 on the surface at South Station. The Washington Street routes use articulated diesel hybrid buses.
The Washington Street corridor was built to replace the Washington Street Elevated, which was used by the Orange Line rapid transit line until 1987. Initial plans called for a light rail branch of the Green Line, but trolleybuses and later CNG buses were substituted. Planning began in 1987 for mass transit to serve the growing Seaport; a new transit tunnel called the South Boston Piers Transitway was chosen in 1989. It was to run from Boylston to World Trade Center via Chinatown and South Station, though the Boylston–South Station section was later deferred as a separate phase. In 1999, the MBTA designated the Washington Street and Transitway projects as the Silver Line, and planned for the Boylston tunnel extension to include a portal to Washington Street for through-running. Service improvements on Washington Street began in 2001. After years of delays, service through the $624 million Transitway began on December 17, 2004.
The connecting tunnel (Phase III) was cancelled in 2010 due to rising costs; a surface route (SL4) was introduced the previous year. The original SL3 route to City Point was discontinued on March 20, 2009. A separate SL3 route to Chelsea – originally planned as part of the cancelled Urban Ring Project – began service on April 21, 2018. Extension of the SL3 route to Sullivan Square is planned. Several other Silver Line extensions have been proposed, as has a conversion of the Washington Street corridor to light rail, but most have not been pursued. The Silver Line has been the target of criticism by riders and transportation planners. Much of the system is missing BRT Standard features such as enforced dedicated lanes, off-vehicle fare collection, sheltered stations, and transit signal priority.
Routes
Waterfront: SL1, SL2, SL3
Three Silver Line services operate from South Station in a dedicated tunnel, the South Boston Piers Transitway, serving the underground Courthouse and World Trade Center stations in the Seaport District then splitting at the Silver Line Way surface station:
- SL1 Logan Airport–South Station
- SL2 Drydock–South Station
- SL3 Chelsea–South Station
During rush hours, additional short turns (designated SLW) are run between South Station and Silver Line Way to increase frequency in the Transitway.
Route SL2 runs on Northern Avenue, then on a one-way loop on Drydock Avenue and Black Falcon Avenue with multiple stops serving the Raymond L. Flynn Marine Park and the Flynn Cruiseport Boston. Buses run clockwise around the loop, with a layover at 23 Drydock Avenue. From Silver Line Way, route SL1 and SL3 buses loop backwards on Haul Road, then cross under Boston Harbor in the Ted Williams Tunnel to East Boston. Route SL1 loops around the Logan International Airport terminals, with stops at the arrivals level of each terminal (including two separate stops at lengthy Terminal B). Route SL3 serves Airport station, follows the Coughlin Bypass Road, and crosses Chelsea Creek on the Chelsea Street Bridge. It then follows a dedicated busway to Chelsea, with intermediate stops at Eastern Avenue, Box District, and Bellingham Square. The three Transitway stops are full rapid transit stations; the Chelsea busway stations have large concrete shelters, while most other surface stops have small shelters.
The Waterfront routes have regular rapid transit fares. Passengers enter through faregates at the three Transitway stations, and pay at the on-board farebox at all other stops. Fares are free when boarding at the Logan Airport stops. Transfer is possible to the Red Line within fare control at South Station. Normal transfers to other routes are available with a CharlieCard; transfers to/from the Blue Line at Airport and the Washington Street routes are available with a CharlieTicket.
The Waterfront routes use 60-foot (18 m) articulated buses with three doors, which provide greater capacity than standard 40-foot (12 m) transit buses. The buses are low-floor and fully accessible, with kneeling bus technology and a wheelchair ramp at the front door. The main Waterfront fleet consists of 45 diesel hybrid buses with extended battery range – sufficient to run through the Transitway on battery power – which were delivered in 2022–2023. A similar extended-battery-range hybrid bus built in 2018 and five battery electric buses built in 2019 are also used. All Silver Line buses are maintained at Southampton Street Garage.
The Waterfront routes previously used dual-mode buses which operated as electric trolleybuses between South Station and Silver Line Way, and as conventional diesel buses on the surface branches. The 32 dual-mode buses, built by Neoplan USA, were delivered in 2004–05 and overhauled from 2014 to 2018. Eight of the buses were funded by Massport and included luggage racks for airport passengers. The dual-mode buses (and overhead lines in the Transitway) proved difficult to maintain and required a time-consuming switch between modes at Silver Line Way. In 2018–19, the MBTA obtained several buses to test alternate options for Waterfront service. 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. On July 31, 2019, the MBTA began using five New Flyer battery electric buses on both Waterfront and Washington Street routes. In November 2020, the MBTA exercised a contract option for 45 additional 60-foot hybrid buses with extended battery range similar to the 2018 test bus to replace the dual-mode Silver Line fleet. The final dual-mode buses were retired in July 2023, ending trolleybus operations in the Boston area.
Washington Street: SL4 and SL5
Two Silver Line routes run between Nubian station (at Nubian Square in Roxbury) and Downtown Boston along Washington Street:
- SL4 Nubian station–South Station
- SL5 Nubian station–Downtown Crossing
These two routes share most of their routing on Washington Street between Nubian Square and Tufts Medical Center, with dedicated lanes for most of the corridor and eight intermediate stops. North of Kneeland Street, the routes run on separate one-way loops. The SL5 runs north on Washington Street to Temple Place (between Downtown Crossing and Park Street, with an intermediate stop at Chinatown. It returns south on Tremont Street, with a southbound stop at Boylston. The SL4 runs north on Washington Street to Chinatown, then east on Essex Street (with a dedicated lane) to a surface stop at South Station. It returns south on Surface Road and Kneeland Street. Most stops have a canopy shelter with seating, maps, and a real-time arrival information display.
The two routes have the same fare as local bus services (lower than rapid transit), with normal transfers with a CharlieCard. In recognition of their role as replacement for the Orange Line, transfers are also available with a paper CharlieTicket (which normally does not allow transfers). 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.
The Washington Street routes use 60-foot (18 m) diesel hybrid articulated buses with three doors. The buses are low-floor and fully accessible, with kneeling bus technology and a wheelchair ramp at the front door. The routes use 21 New Flyer buses delivered in 2016–17 (part of a 44-bus order also used on routes 28 and 39), which replaced the original compressed natural gas (CNG) buses, plus three similar hybrid buses delivered in 2010. All Silver Line buses are maintained at Southampton Street Garage.
History
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 Braintree, 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 Harvard. In 1948, the legislature authorized the city to issue $19 million in bonds (equivalent to $203 million in 2025) 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.
The northern part of the new corridor was about 1⁄2 mile (0.80 km) west of Washington Street, so the MBTA (which had replaced the MTA in 1964) began planning in 1978 for a replacement service between Dudley Square and Downtown Boston. 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. Local opinion favored the temporary retention of the northern portion of the Elevated until a permanent replacement could be built. However, the MBTA closed the Elevated and instead upgraded the route 49 bus from a feeder route to a more frequent trunk route.
In 1989, the MBTA announced that trolleybuses would be used on Washington Street, operating on 4-minute headways at peak hours. 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. 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. Environmental documentation was filed in 1998, and construction began in 2001. The project cost $27.3 million, with major elements including $10.9 million for the 17 new buses, $10.9 million for road work, and $2.6 million for shelters. Planning and construction were combined with a necessary repaving of Washington Street, reducing costs. Intended for the route to equal the service quality of light rail, the MBTA branded it as the "Silver Line" and designated it equally to the existing MBTA subway lines on maps. The Silver Line name was introduced in 1996.
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. Although the MBTA considered other stop locations, most of the final stops were at existing route 49 stops. The conversion to the Silver Line occurred in several steps. In December 2001, the MBTA opened a contraflow bus-only lane on Washington Street between Marginal Road and East Berkeley Street, allowing southbound buses to use Washington Street. This eliminated a longer outbound routing via Surface Road, and allowed an extension to a new downtown terminal at Temple Place (between Park Street and Downtown Crossing stations). Service frequency was also increased at that time. 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. In November 2002, a twelfth stop at Worcester Square was restored. The 40-foot (12 m) were replaced by 60-foot (18 m) buses in August 2003, and service frequency was again increased. On January 31, 2005, the route was the first MBTA bus route to receive new automated fare collection equipment. By 2005, ridership was double that of route 49, but early decreases in travel time were cancelled out by longer dwell times.
In February 2020, the MBTA agreed to change the name of Dudley Square station to Nubian, following the December 2019 renaming of the square itself. The station renaming took effect in June 2020. In May 2022, the MBTA released a draft plan for a systemwide network redesign. The draft called for the SL4 and SL5 to be combined into a single Nubian–South Station route to provide simpler and more consistent downtown service. The November 2022 draft network plan kept the same proposal. An additional SL4 stop (Chinatown Gate) on Surface Artery at Kneeland Street in Chinatown was added on August 19, 2022, while the Orange Line was closed for maintenance work; it was retained after the closure.