BEIJING, Dec. 10 (Xinhua) -- Total investment in Beijing's subway system is set to approach 80 billion yuan (10.8 billion U.S. dollars) by 2010, and the city's underground network is expected to be the world's largest as of 2015.

    The financing includes 71.5 billion yuan approved by the municipal government for public transport during the 11th five-year plan period, which ends in 2010, the Beijing Morning Post reported on Monday.

    The huge investment will boost construction of rail lines, which are expected to total 561 km by 2015 with a capacity of 9 million passengers daily.

    By that time, rail service will extend from the downtown area to many key locations on the city fringes, including the Olympic Park and seven suburban satellite cities.

    Subway transport is expected to account for 50 percent of all public transit use within eight years, greatly easing pressure on the roads.

    Five new subway routes -- Line 6, the second phases of Line 10 and Line 8, and the Yizhuang and Daxing lines -- with a total length of nearly 140 km began construction in the city on Saturday. All are to be completed by 2015.

    Currently, the first phases of Line 10 and Line 8, as well as a 28-km line linking downtown to Beijing Capital International Airport, are under construction.

    The capital now has Line 1, Line 2, Line 5, Line 13 and the Batong line in operation, carrying about 2.3 million passengers each day. Their combined length is 142 km.