M (SEPTA Metro)

The M, formerly known as the Norristown High Speed Line (NHSL), is a 13.4-mile (21.6 km) interurban light rapid transit line in the SEPTA Metro network, running between the 69th Street Transportation Center in Upper Darby and the Norristown Transportation Center in Norristown, Pennsylvania. Service is operated by the Suburban Transit Division of the Southeastern Pennsylvania Transportation Authority. Originally the Philadelphia and Western Railroad line (which is why the line is referred to by locals as "the P&W"), the line runs entirely on its own right-of-way. By 2020, the M had an average weekday ridership approaching 11,000 passengers.

The M is unique in its combination of transportation technologies. Originally chartered as a Class I (steam) railroad, the line is fully grade separated, collects power from a third rail, and has high-level platforms common to rapid transit systems or commuter rail systems such as New York City's Long Island Rail Road and Metro-North Railroad, but has onboard fare collection, mostly single-car operation, and frequent stops more common to light rail systems. Previously, the M was considered to be a light rail line, according to a 2008 SEPTA budget report; however, the line is currently considered an interurban heavy rail line, according to a 2009 SEPTA business plan, and subsequent capital budgets. It has also been categorized by the American Public Transportation Association as "Intermodal High Speed rapid rail transit".

The service is similar in design and use to the Mattapan Line in Boston. The purple color-coded line was formerly known simply as Route 100, changed to the Norristown High Speed Line in September 2009, and to its current name in 2025.

There have been multiple collisions and incidents on the line. The first recorded crash occurred near an Ardmore stop on January 26, 1987 injuring 19. The operator tested positive for drugs and was convicted on reckless endangerment. Another crash occurred on July 6, 2012 between Beechwood-Brookline and Penfield stations when the cars detached and came back together, injuring two. In August 2017, there was a crash involving an unoccupied railcar at the 69th Street Terminal that injured more than 40 people. As a result, the maximum operating speed on the line was decreased to 55 mph (89 km/h) from 70 mph (110 km/h).