The B, formerly known as the Broad Street Line (BSL), 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 Transit 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. The trains of the B run underneath Broad Street 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 L, though Center City Philadelphia is also served by five 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). B 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 (B), 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^{[by whom?]} at $102 million.^{[citation needed]}

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.