History: The New York City Subway, United States

The New York City Subway is the fourth busiest rapid transit rail system in the world in annual ridership, after Tokyo's, Moscow's, and Seoul's rapid transit systems, and the busiest in the Americas. It is one of the four systems in the U.S., along with portions of the Chicago 'L' system, PATH, and PATCO, to offer service 24 hours a day, 365 days a year.
Overview
The system's stations are located throughout the boroughs of Manhattan, Brooklyn, Queens and the Bronx. Staten Island has a rail line, the Staten Island Railway, which opened in 1860 and uses R44 subway cars, but has no links to, and is not officially considered part of the subway, though it has been included on all official Subway Maps since 1998.