Complex sentence: It is the second largest metro system in North America after the New York City Subway, and in 2011 the system served 1.487 billion passengers, placing it as the ninth highest ridership in the world.
Split into simpler sentences: It is the second largest metro system in North America after the New York City Subway. In 2011 the system served 1.487 billion passengers, placing it as the ninth highest ridership in the world.