The Market–Frankford Line (MFL), or the Blue Line) is one of three rapid transit lines in Philadelphia, Pennsylvania; it and the Broad Street Line are operated by SEPTA, and the PATCO Speedline is operated by PATCO. The Market–Frankford Line runs from the 69th Street Transportation Center in Upper Darby, just outside of West Philadelphia, through Center City Philadelphia to the Frankford Transportation Center in Near Northeast Philadelphia. From 2024 onwards, this line will be rebranded as the L line.

Rapid transit or mass rapid transit (MRT), also known as metro, is a type of high-capacity public transport that is generally built in urban areas. A rapid transit system that primarily or traditionally runs below the surface may be called a subway, tube, or underground. Unlike buses or trams, rapid transit systems are railways, usually electric, that operate on an exclusive right-of-way, which cannot be accessed by pedestrians or other vehicles. They are often grade-separated in tunnels or on elevated railways.