The New York City Subway is the largest subway system in the world when measured by number of stations (), and the eighth-largest when measured by annual ridership (1.76 billion passenger trips in 2015). It is the second-oldest subway system in the United States after the rapid transit system in Boston. In 2002, an average 4.8 million passengers used the subway each weekday. During one day in September 2005, 7.5 million daily riders set a record for ridership. In 2013, the subway delivered over 1.71 billion rides, averaging approximately 5.5 million rides on weekdays, about 3.2 million rides on Saturdays, and about 2.6 million rides on Sundays. Ridership consistently increased in the early 21st century, partly because of the subway's energy efficiency. Life in New York City is so dependent on the subway that the city is home to three of only five 24-hour subway systems in the USA. The city's subway services run through all boroughs except Staten Island, which is served 24/7 by the Staten Island Railway.