The New York City Subway System (Building America: Then and Now)

Teeming with a population of 3.5 million at the end of the 19th century, the island of Manhattan couldn't meet the city's demand for rapid transit with its horse-drawn trolleys and elevated train lines. New York City needed a subway system. After four years of digging and diverting miles of utilities and tunneling under the Harlem River, the city's residents celebrated a new era in mass transit on October 27, 1904, with the opening of a nine-mile subway route.