The Beijing Subway opened in 1969 and serves Beijing and the surrounding suburbs. Just before the 2008 Beijing Olympic games began, it was further developed at a cost $7.69 billion. The expansion project has taken the length of the subway station to about 480 km.

With almost a dozen lines and 123 stations currently in operation and an average passenger strenght of 3.4 million per day, the Beijing Subway is the busiest in mainland China.

Image: Passengers wait for the train in the subway station of the new airport line connected to Beijing airport's Terminal 3 (T3) on July 29, 2008, which was just completed and opened on July 19 ahead of the 2008 Olympic Games. | Photograph: Frederic J Brown/AFP/Getty Images.