Mexico DF Metro

Some facts and figures : Mexico City's Metro network is the 9th biggest in the world when measured by number of stations (175), 5th biggest by number of passenger rides per year (1.442 billion), and 8th longest by length (201.7 kms) and, by my own estimation, number one when it comes to the number of sellers patrolling the system.

From Wikipedia:
The Mexico City Metro (formally: Sistema de Transporte Colectivo Metro or STC Metro) provides metro service to the central and northern area of the Mexican Federal District and surrounding municipalities in the State of México. In 2004 the system served an average of 3.9 million passengers per day, the fifth highest ridership in the world.

The first Metro line with 16 stations was opened to the public in 1969. It has expanded since then in a series of fits and starts; it currently comprises eleven lines and 177 kilometres of passenger track. Trains feature rubber tires instead of traditional steel wheels, decreasing noise and making the system tolerant of Mexico City's unstable soils.

During rush hour, the trains are constantly serviced by unlicensed vendors selling pirate DVDs and music CDs, as well as large variety of other products. Some lines designate the first two cars of the train for women and children only, though this is only strictly enforced at rush hours.

The Metro has 175 stations, 24 of which serve two or more lines. It has 106 underground stations (the deepest of which are 35 metres below street surface); 53 surface stations and 16 elevated stations. Eleven stations are located in the State of Mexico, while the rest are within the limits of the Distrito Federal.