In the mid 1800s, New York City was one of the most crowded places on earth. The congested streets and pokey transportation system were a source of constant complaint. On March 24, 1900, ground was broken for the Big Apple's subway; the Interborough Rapid Transit Line opened four years later, running more than 26 miles of underground track at the speed of 35 miles per hour. Soon thousands in the city were "doing the subway."