In the 19th century, New York City was a burgeoning industrial and commercial metropolis - the largest city in the United States and second largest in the world. As the city's population increased, people began to call for construction of an underground railway. Many unusual engineering challenges had to be overcome, not the least of which was construction in a dense urban area. After lengthy legal battles over property rights and the debt limit of the city, ground was broken on March 24, 1900.
When the subway opened in 1904, it was operated by the private Interborough Rapid Transit Company (IRT). In 1940 the City merged the IRT with the Brooklyn Manhattan Transit system (BMT) and the Independent Subway System (IND), creating a single municipal agency. Today the New York subway system carries over 1 billion riders each year.