In 2013, the subway delivered over 1.71 billion rides,[37] 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.[38][39][40][41][42] 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.[43] The city's 28 subway services run through all boroughs except Staten Island, which is served 24/7 by the Staten Island Railway.