New York City, with a population of 18.6 million, is the biggest commuter city with an underground train system in North America. The sprawling system has an average weekday ridership of more than 5.6 million, according to the city’s Metropolitan Transportation Authority. The subway system has 472 stations, the most of any system in the world, according to the MTA. It has 665 miles of track. Women-only cars were considered in 1909, five years after the subways opened, but the idea did not take hold.