Silver Line (MBTA)

The Silver Line is a system of bus routes 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 2023^[update], weekday ridership on the Silver Line was 27,000.

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.

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:

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.