The Broad Street Line (BSL), currently rebranding as the B, is a rapid transit line in the SEPTA Metro network in Philadelphia, Pennsylvania, United States. The line runs primarily north-south from the Fern Rock Transportation Center in North Philadelphia through Center City Philadelphia to NRG station at Pattison Avenue in South Philadelphia; the latter station provides access to the stadiums and arenas for the city's major professional sports teams at the South Philadelphia Sports Complex, about a quarter mile away. It is named for Broad Street, under which the line runs for almost its entire length.

The line, which is entirely underground except for the northern terminus at Fern Rock, has four tracks in a local/express configuration from Fern Rock to Walnut-Locust and two tracks from Lombard-South to the southern terminus at NRG station. It is one of only two rapid transit lines in the SEPTA Metro system overall alongside the Market–Frankford Line, though Center City Philadelphia is also served by four stations of the PATCO Speedline rapid transit line which runs from Center City Philadelphia through Camden, New Jersey to Lindenwold, New Jersey. With about 115,000 boardings on an average weekday in 2019, it is the second busiest route in the SEPTA system.

The line and its trains were leased to SEPTA in 1968 after it assumed operation of the city transit systems from the former Philadelphia Transportation Company (PTC). Broad Street Line subway cars bear both the SEPTA logo and the seal of the City of Philadelphia to reflect the split ownership-operation arrangement.

Service on the northern half of the Broad Street Line, between City Hall and Olney Avenue, opened on September 1, 1928. While the original subway tunnel had been finished to just north of the present-day Lombard-South station, service to the Walnut-Locust station did not begin until 1930, and the Lombard-South station entered service in 1932. Service from that point south to Snyder Avenue began on September 18, 1938. Service to a new park-and-ride station built next to the Fern Rock shops began in 1956, and the line was extended further south to Pattison Avenue in 1973 to serve the recently completed Sports Complex.

The total cost of the original segment, "Olney Avenue to South Street," was stated at $102 million.

Although the Broad Street Line was originally planned in the 1920s to be a 4-track facility for its entire length (Fern Rock portal to Snyder), the tunnel was built with provision for 4 tracks only from the portal to just north of Lombard-South. At the time of opening, the outer 2 tracks were built along this length, whereas the inner 2 express tracks were built only in two sections, from the Fern Rock portal/shops to just south of Olney, and from Girard to their terminus just north of Lombard South. To close the gaps, the two inner express tracks were laid from Erie to Girard in 1959, and again from Olney to Erie in 1991.

From Lombard-South station south to Snyder, the tunnel was constructed differently – only the eastern half of the line was built. The track currently used for southbound trains is actually the northbound express track. The extension in 1973 to Pattison station (now called NRG station) continued this arrangement. Space exists under the western half of Broad Street for the construction of the western half of the tunnel, which would include the remaining 2 tracks and additional island platforms for southbound local and express trains. The resulting infrastructure would match the configuration built in the northern half of the line.

Provisions for flying junctions exist in the tunnels at three locations: north of Olney station, north of Erie station, and between Tasker-Morris and Snyder stations. These were to connect to planned but never built extensions to the north, northeast, northwest and southwest. Tracks were laid in the upper levels of the flying junctions north of Olney and Erie; these have been used over the years to store out-of-service trains and as layover points for express and Ridge Spur trains.

The NRG Station contains a lower level platform (very narrow compared to the very wide upper level platform), built to accommodate additional trains for large crowds at sporting events. Seldom used for passenger service in recent years, these tracks are most often used to store rolling stock and work trains.

Two of the Broad Street subway system's stations have been closed. The Spring Garden station on the Ridge Avenue spur line was closed in 1989. The Franklin Square station on the PATCO route was closed in 1953, reopened in 1976 for the US Bicentennial, then closed again in 1979. It is scheduled to reopen (after a major renovation) in 2024.

The Broad Street Line is one of only two rapid transit lines in the United States outside of New York City to use separate local and express tracks for a significant length, the other being Chicago's North Side Main Line from Armitage north, used by Purple Line express trains.

During early 2020, the line operated "Lifeline Service" due to the COVID-19 pandemic in Pennsylvania. From April 2020, trains bypassed the Logan, Wyoming, Susquehanna–Dauphin, Fairmount, Spring Garden, Chinatown, Lombard–South, and Tasker–Morris stations. All stations were reopened by July 2020.

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  Proposed new icon for the Broad Street Line under the SEPTA Metro wayfinding project
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  Proposed new icon for the Broad Street Local Line under the SEPTA Metro wayfinding project
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  Proposed new icon for the Broad Street Express Line under the SEPTA Metro wayfinding project
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  Proposed new icon for the Broad Street Spur Express Line under the SEPTA Metro wayfinding project

In September 2021, SEPTA proposed updating wayfinding across the SEPTA Rail Transit network. Part of the proposal, SEPTA proposed rebranding their rail transit service as "SEPTA Metro", in order to make the system easier to navigate. Under this proposal, services along the Broad Street Line will be rebranded as the "B" lines with an orange color. Each service utilizing the trunk would receive a numeric suffix. Local service would be known as the B1 Broad Street Local, the express and special service as the B2 Broad Street Express and B2 Express Sport Special, and the Broad–Ridge Spur as the B3 Broad–Ridge Express.

Following public feedback, SEPTA revised the Wayfinding Master Plan. Rather than being referred to as the B Lines, the current Broad Street Line would become the B Line. Express and special services would be consolidated into the B2 Broad Street Express, with signage letting riders know whether a train terminates at Walnut–Locust or NRG station. Additionally, SEPTA stated they would pilot neighborhood maps in stations and prioritize the deployment of real-time information signage and on mobile apps.

Both the City of Philadelphia and SEPTA have studied extending the Broad Street Line along Roosevelt Boulevard, in order to serve a growing population in the northeast section of the city. The city government's archives contain a survey report, prepared in 1948, discussing a need for an extension of the Broad Street line from Erie Avenue to the vicinity of Pennypack Circle (see Roosevelt Boulevard). Subway car destination signage even included station and terminus names for major streets along Roosevelt Boulevard such as Rhawn Street, in the newer "South Broad" cars. An expansion into another part of the City could better use the capacity of the four-track trunk line.

In 1964, the city proposed a nine-mile (14 km), $94 million extension of the Broad Street line along Roosevelt Blvd. in conjunction with a new Northeast Expressway to be built by the Pennsylvania Department of Transportation. Development was limited to the building of one subway station by Sears, Roebuck and Company in 1967, at its complex on Roosevelt Boulevard at Adams Avenue, at the cost of $1 million, in anticipation of future service. This station was destroyed when the facility was demolished in October 1994. Ultimately the Northeast Expressway was never built, due to lack of funds, and the subway extension remained a paper concept.

On September 10, 1999, SEPTA filed a Notice of Intent to prepare an Environmental Impact Statement for the Northeast Extension with the EPA. In December 2001, the Philadelphia City Planning Commission supported extending the Broad Street Line along Roosevelt Blvd. to Bustleton Avenue, where it would be joined by the Market–Frankford Line, extended from its Frankford terminal (now the rebuilt Frankford Transportation Center). The estimated cost had ballooned to $3.4 billion.

Currently, the Broad Street Line terminates southbound at NRG station at Pattison Avenue and three major stadiums. With the redevelopment of the Philadelphia Naval Yard directly to the south, a Health Impact Assessment report was issued in March of 2012 to determine if extending the line to the Naval Yard would be a viable option for commuters. It determined that extending the line to the Naval Yard would more than halve the number of private cars commuting back and forth, with the remainder taking the proposed subway line and/or using a bicycle sidepath. The HIA recommends making an extension of the Broad Street Line a priority, and recently, the extension has garnered much support.

A report in the 1940s proposed an extension of the Locust St. subway to West Philadelphia. This line would have run under one of the streets presently served by the subway-surface system. Presumably, the current subway-surface lines would have been converted to bus operation and would have been used to feed this line. It appears that this proposal was replaced by the extension of the subway portion of the subway-surface system in the 1950s.

The same report also proposed a northwest extension. This would have branched off at the North Philadelphia station and would have taken over the Pennsylvania Railroad's Chestnut Hill Branch which is still operated today as part of the SEPTA Regional Rail system as the Chestnut Hill West Line.

The first set of rail cars for the Broad Street subway was the B-1 cars built in 1926–27 by the J.G. Brill Company. The Pressed Steel Car Company supplied an additional set in 1938 collectively known as the B-2's. The JG Brill Company also built and delivered 26 deluxe art-deco streamlined subway cars to the Delaware River Joint Commission in early 1936 for use on its Bridge Line from 8th and Market into Camden, NJ via the Benjamin Franklin Bridge. These cars were designed to be compatible with the other Broad Street cars, and could run in multiple with them. After the Bridge Line became part of the PATCO Lindenwold Hi-Speedline in late 1968, 23 of these former "Bridge Line" cars were sold to the City of Philadelphia in 1969 to be used on the Broad Street subway, and were designated as the B-3's, until they were retired by early 1984.

The first set has had the second longest lifespan of any subway car in Philadelphia, after that of the Market Street cars built for what ultimately became the Market–Frankford Line. Although the line was a host for the UMTA's State of the Art Car program, real replacements for the Broad Street cars did not come until late 1982, when SEPTA introduced new "B-IV" cars built by Kawasaki, which are currently the only cars operating the line. The cars are 67ft 6in long, 10ft 1.5in wide, and 12ft 3in tall.

• A small number of B-1, B-2, and B-3 historic cars remain stored in derelict condition within Fern Rock yard.
• One B-1 car was sent and sold to the Trackside Brick Oven Pizzeria in Wallingford, Connecticut.
• One B-1 car (#55) is present at the Illinois Railway Museum.
• One B-3 car (#1009) is partially restored at the Rockhill Trolley Museum.
• Two B-3 cars (#1018, #1023) are at the Seashore Trolley Museum.

A local trip along the entire line takes about 35 minutes. Trains run from approximately 5:00 am to 1:00 am, with a timed transfer at 12:30 am at City Hall station to connect with the Market Frankford Line based on final trains. The Broad Street Owl bus service replaces the subway throughout the night Monday through Friday mornings, stopping at the same locations as the subway trains. The line itself ran 24 hours a day until it was eliminated in 1991; it was reinstated on June 20, 2014, for Friday and Saturday overnights only on a trial basis. It was made permanent on October 8, 2014, due to the line successfully carrying an extra 10,000 riders on the Broad Street Line during the weekend overnight periods. This was eliminated again in 2020 due to the COVID-19 Pandemic.

The local portion of the Broad Street Line carries a headway of 8 minutes or less during the daytime all day weekdays, 10–12 minutes all day on weekends and major holidays, and 12 minutes in the evenings. Weekend night service consists of a 20-minute frequency, while owl bus service early weekday mornings utilize a 15-minute frequency. The express portion of the line ranges from seven minutes during peak hours to 12 minutes off-peak, while the Broad Ridge Spur ranges from 7 minutes during peak hours to 20 minutes off-peak.

Service	Start Time	End Time
Northbound train	4:52 am	12:55 am
Southbound train	4:45 am	12:43 am
Northbound night owl bus	12:22 am	5:35 am
Southbound night owl bus	12:14 am	5:34 am
Northbound Broad–Ridge Spur (Monday–Friday)	5:45 am	9:15 pm
Southbound Broad–Ridge Spur (Monday–Friday)	5:24 am	8:48 pm
Northbound Broad–Ridge Spur (Saturday)	6:40 am	9:26 pm
Southbound Broad–Ridge Spur (Saturday)	6:17 am	9:03 pm

A two-track spur of the Broad Street Line, known as the Broad–Ridge Spur, diverges from the main line at Fairmount. Originally known as the Ridge–8th Street subway, the line follows Ridge Avenue, southeastward from the intersection of Broad Street, Ridge and Fairmount Avenues to a two-level junction beneath 8th and Race Streets, where tunnels leading to and from the Benjamin Franklin Bridge to Camden connect to it, then proceeds south under 8th Street. At its southern terminus at 8th and Market streets, passengers may transfer to the Market–Frankford Line and the PATCO Speedline. The spur operates Mondays through Saturdays from 6 am to 9 pm, running two-car trains (though platforms can fit five cars).

Ridge Spur service to 8th and Market streets began on December 21, 1932. As part of that project, a tunnel shell running south under 8th Street then west under Locust Street to 18th Street (reusing parts of the never-completed Center City loop constructed in 1917) was completed in 1933 but not outfitted for service. Bridge Line service from 8th and Market to Camden began on June 7, 1936, sharing the Ridge Spur platforms at 8th and Market and splitting off from the Ridge Spur just south of Chinatown station.

Beginning in June 1949, Ridge Spur and Bridge Line trains were through-routed at 8th and Market. The unused Locust Street tunnel was completed on February 15, 1953; Bridge Line trains were extended to a new terminus at 15th–16th Street station with two intermediate stops, while Ridge Spur trains reverted to running between 8th Street and Girard. In January 1954, due to low ridership, off-peak service and Saturday again began operating between Girard and Camden, with a shuttle train operating between 8th and 16th stations. Sunday service was suspended at that time due to minimal usage. Ridge Spur service was suspended from August 23 to 27, 1968, as tracks were switched to a new upper-level terminal platform at 8th Street station to allow conversion of the 8th–Locust Street subway into the Lindenwold High-Speed Line (PATCO Speedline).

The Ridge Spur was closed from February 1981 to September 6, 1983, during construction of the Center City Commuter Connection. Spring Garden station, by then exit-only, was closed on September 10, 1989, due to safety concerns. Never drawing high ridership, the spur has been proposed for closure on several occasions. The 2014 closure of the Gallery Mall, adjacent to 8th and Market station, caused ridership on the spur to drop by 25%.

All stations are located in the city of Philadelphia, Pennsylvania. Stations on the Broad–Ridge Spur are shaded in gold. Special extra service for sports and entertainment events makes all express stops between Fern Rock to Walnut–Locust and then continue express to NRG.

  Local (B1)   Express (B2)   Broad–Ridge Spur (B3)

Neighborhood	Station	B1	B2	B3	Connections	Weekday Ridership (2018)	Notes
Fern Rock	Fern Rock Transit Center	●	●	●	SEPTA Regional Rail:   Warminster Line,   West Trenton Line,   Lansdale/​Doylestown Line SEPTA City Bus: 4, 28, 57, 70	4,498
Logan	Olney Transit Center	●	●	●	SEPTA City Bus: 6, 8, 16, 18, 22, 26, 55, 80, L	16,591	Serves Einstein Medical Center Philadelphia and La Salle University
	Logan	●	│	│	SEPTA City Bus: 16, J	2,452
	Wyoming	●	│	│	SEPTA City Bus: 16 SEPTA Trackless Trolley: 75	2,087
Hunting Park	Hunting Park	●	│	│	SEPTA City Bus: 1, 16, 53, R	3,006
	Erie	●	●	●	SEPTA City Bus: 16, 23, 53, 56, H, XH	7,750	Serves Temple University Hospital
Glenwood	Allegheny	●	│	│	SEPTA City Bus: 4, 16, 60	3,842	Serves Maurice H. Kornberg School of Dentistry
	North Philadelphia	●	│	●	Amtrak: Northeast and Keystone Corridor services (at North Philadelphia) SEPTA Regional Rail:   Trenton Line,   Chestnut Hill West Line (at North Philadelphia),   Lansdale/​Doylestown Line,   Manayunk/​Norristown Line (at North Broad) SEPTA City Bus: 4, 16, 54	4,168
Cecil B. Moore	Susquehanna–Dauphin	●	│	│	SEPTA City Bus: 4, 16, 39	3,392	Originally named Dauphin-Susquehanna
	Cecil B. Moore	●	│	│	SEPTA City Bus: 3, 4, 16	7,375	Serves Temple University, originally Columbia Avenue
Francisville	Girard	●	●	●	SEPTA Metro: Route 15 Trolley SEPTA City Bus: 4, 16	4,009
	Fairmount	●	│	●	SEPTA City Bus: 4, 16, 61	2,156
Callowhill	Spring Garden			│			Closed since 1989
Chinatown	Chinatown			●	SEPTA City Bus: 47, 47m, 61 (all south) NJ Transit Bus: 317, 400, 401, 402, 404, 406, 408, 409, 410, 412, 551 (at Vine St & 8th St)	322	Originally named Vine
Market East	8th & Market			●	SEPTA Metro: Market–Frankford Line DRPA: PATCO Lindenwold Line SEPTA City Bus: 17, 33, 38 (west), 44, 47 (south), 47m (south), 61, 62 NJ Transit Bus: 313, 315, 316, 317, 400, 401, 402, 404, 406, 408, 409, 410, 412, 414, 417, 551, 555	2,254	Originally named Market Street
Chinatown	Spring Garden	●	●		SEPTA City Bus: 4, 16, 43	7,462	Serves the Community College of Philadelphia
	Race–Vine	●	●		SEPTA City Bus: 4, 16, 27 NJ Transit Bus: 317, 400, 401, 402, 404, 406, 408, 409, 410, 412, 551	3,226	Serves the Pennsylvania Convention Center
Center City	City Hall	●	●		SEPTA Regional Rail: all lines (at Suburban Station) SEPTA Metro: Market–Frankford Line, Subway–Surface Trolleys (at 15th Street) SEPTA City Bus: 4, 16, 17, 27, 31, 32, 33, 38, 44, 48, 62, 78 SEPTA Suburban Bus: 124, 125	30,506	Located in the Downtown Link concourse
Washington Square West	Walnut–Locust	●	●		DRPA: PATCO Lindenwold Line (at 15–16th & Locust) SEPTA City Bus: 4, 9, 12, 16, 21, 27, 32, 38 (east), 42	7,633	Serves the Kimmel Center and Academy of Music
	Lombard–South	●			SEPTA City Bus: 4, 27, 32, 40	2,915	Serves University of the Arts, Peirce College, and Graduate Hospital
South Philadelphia	Ellsworth–Federal	●			SEPTA City Bus: 4, 64	3,715	Serves the Italian Market
	Tasker–Morris	●			SEPTA City Bus: 4, 29	4,505
	Snyder	●			SEPTA City Bus: 4, 37, 79	5,500	Serves Methodist Hospital
	Oregon	●			SEPTA City Bus: 4, 7, 45, 68, G	4,045	Serves Marconi Plaza
	NRG	●			SEPTA City Bus: 4, 17	1,541	Serves the South Philadelphia Sports Complex, originally named Pattison and later AT&T

• Broad Street Line on SEPTA website
• World.nycsubway.org: Philadelphia: SEPTA Broad Street Subway