Answer:  The Market-Frankford Line

The Market-Frankford Line (MFL) opened 105 years ago this month in March 1907, with the Norristown High Speed Line (then called the Philadelphia and Western Railroad) starting a few months later. The Broad Street Line opened along its northern portion in 1928, and PATCO started service in 1969.

Underground rapid transit, “the subway,” is a transportation model that first started in London in 1863. New York City and Boston developed their own rapid transit systems in the late nineteenth and early twentieth centuries with New York City’s first line opening in 1904. The urban transportation systems in the United States as a whole changed rapidly during the late 1800s from horsecars, which were replaced by steam locomotives and steam-powered underground cables due in part to horses’ capacity for disease and injury as well as the uninviting smell and noise. Steam-power in turn became obsolete due to the efficiency and speed electric-powered rail systems.

Since the founding Philadelphia’s Market-Frankford Line over 100 years ago, it has undergone changes; including the demolition of a western segment of the elevated rail, which was replaced by a subway, along with revitalization projects that have rebuilt the Frankford side of the line. Today the MFL is the busiest subway in Philadelphia.

Images:  Building the Market Street subway, photograph (1906)