Shanghai

• Although construction on Shanghai, China's subway system began only in 1990, the city has the longest subway system in the world as of 2011. The Shanghai Metro offers 261 miles of lines. It has 282 stations and 11 lines and carries more than five million passengers per day. According to Railway Gazette International, Shanghai has an additional 87 miles of track under construction and scheduled for completion in 2012, and 186 more miles planned to be put in service by 2020.
  
  

London

• The London Underground -- popularly known as "the tube" -- is the world's second-longest subway network with 254 miles of track, as of 2011. The London Underground was inaugurated in 1863, making it the world's oldest. The network has 275 stations and serves three million passengers every day, according to the Transport for London website.
  
  

New York City

• New York City has the world's third-longest subway system. At 229 miles long, the city's underground rail network has grown dramatically from its launch on Oct. 27, 1904 as a 9.1-mile line that spanned 28 stations from City Hall to 145th Street and Broadway. According to the Metropolitan Transportation Authority, which operates the subway, New York claims the fourth-largest ridership in the world with 1.563 billion passengers annually, behind Tokyo, Moscow and Seoul, respectively. The longest ride without a change of trains on New York's subway is the A line from 207th Street in Manhattan to Far Rockaway in Queens -- a trip of more than 31 miles.
  
  

Beijing

• The Beijing Subway spans 208 miles, making it the fourth-longest system in the world. China's capital city began construction on the subway in 1965, and it operates 14 lines and 198 stations as of 2011. The network is undergoing an expansion and is expected to be 261 miles long by 2012. By 2015, the Beijing Metro is projected to be the longest in the world, with a total of 19 lines and 349 miles of track in operation, states Railway-Technology.com.