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. In the century to come, the New York subway would grow and expand to a system that runs 24 hours a day, seven days a week, with 6,400 cars, 468 stations, a daily ridership of 4.5 million, and 842 miles of track - longer than the distance from New York to Chicago. Politics, graffiti, and unbelievable construction challenges combined to make the building and running of the New York subway system one of the America's greatest civic undertakings.