B (SEPTA Metro)

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.