Beijing’s 5th subway line is scheduled to open in 20th September, this is Beijing’s first north-south line and runs through some important parts of the city center. Before the 5th line be opened, there are four lines of Beijing subway now, I picked some infomation from wikimedia about it:

Beijing’s subway system was the first such system nationwide. Construction works began on July 1, 1965, as part of the military defense systems after the Sino-Soviet split. Three major cities - Beijing, Shanghai and Shenyang were proposed to build subway systems in the 1960s, but only the Beijing Subway was built.

The first line of Beijing Subway connects Beijing railway station to Pingguoyuan (I’m here), was first operated on October 1, 1969.

On September 20, 1984 the rest of Line 2 opened, and the Beijing Subway henceforth operated as two lines. On December 12, 1992, Line 1 was extended eastwards from Fuxingmen to Xidan. On September 28, 1999 Line 1 was further extended from Xidan to Sihuidong. The entire passageway was opened on June 24, 2000, thus allowing trains to go directly from Pingguoyuan to Sihuidong.

Line 13 was opened in two sections, the western section to Huoying on September 28, 2002, and the eastern section on January 28, 2003. In the meantime, the Batong Line, built as an extension to Line 1, was opened as a separate line on December 27, 2003.

The Beijing Subway currently has 4 lines: Line 1, Line 2, Line 13 and Batong Line. As part of the preparations for the 2008 Summer Olympics, Beijing has pledged to expand its public transportation network. Beijing projects expansion of its subway network from the current 114 km to 481.2 km by 2015 (around 200 km by 2008).

Beijing plans to invest 63.8 billion yuan (7.69 billion US dollars) in future projects between 2002 and 2008. The Beijing Subway is now a market-oriented operation and is jointly funded by government and investors. The remainder will be covered by low interest bank loans, mostly provided by the Big Four banks in China.

you can getting more details on wikimedia:
http://en.wikipedia.org/wiki/Beijing_Subway