The Market–Frankford Line (MFL) (also called the Market–Frankford Subway–Elevated Line (MFSE), the Market–Frankford El (MFE), the El (), 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. With more than 170,000 boardings on an average weekday, it is the busiest route in the SEPTA system. The line has both elevated and underground portions along its full length. Route The Market–Frankford Line begins at 69th Street Transportation Center in Upper Darby. The MFL heads east at ground level and passes north of the borough of Millbourne. From there, it enters West Philadelphia and is elevated over Market Street until 46th Street, where it curves north and east and then descends underground via a portal at 44th Street. At 42nd Street, the tunnel returns to the alignment of Market Street. At 32nd Street, the tunnel carrying the SEPTA subway–surface trolley lines joins the MFL tunnel. The MFL tracks are in the center and the trolley tracks are on the outside. 30th Street consists of an island platform between the two innermost tracks for Market–Frankford Line trains, and outboard "wall" platforms for subway–surface route 10, 11, 13, 34, and 36 trolleys. After passing beneath the Schuylkill River, the next stop to the east for Market–Frankford Line trains is at 15th Street; subway–surface trolleys also have stations at 22nd Street and 19th Street. 15th Street is the central interchange station for the MFL, subway–surface trolleys, and Broad Street Line. The subway–surface trolley tracks end in a loop beneath Juniper Street at Market just after crossing above the Broad Street Line. Though it now tunnels in a straight line directly beneath Philadelphia City Hall, prior to 1936, the original MFL trackage between 15th and 13th Street stations separated and looped around the foundation of City Hall (eastbound trains passed around the south side and westbound trains passed around the north side). Parts of that original alignment are now used by subway–surface cars as they pass south of City Hall en route to 13th Street station (as well as the bridgework in the ceiling of the southbound platform of the City Hall