Beijing's subway system will replace London's Underground as the world's longest metro system by 2020, according to a construction plan drafted by the communications commission for the city's public transport system.

The Beijing metro will extend 561 kilometers along 19 lines, and the new lines that are being planned will service the major corners of the city.

Fifteen of the new lines will serve urban areas, while the other four will serve Beijing's suburbs. Metro lines 1 and 2 go around Tian'anmen Square, the traditional city center.

Lines 4, 5 and 10 are still under construction but are expected to be operational in time for the 2008 Olympics.

Line 4, which is will span 28.16 kilometers, will connect Fengtai with Haidian. Fengati is in the extreme south while Haidian is in the northwest.

Line 5 will also create a short-cut between the "developed" north and the "developing" south, from Changping to Fengtai districts.

A part of Line 10 will be used to ease traffic congestion due to the Olympic Games, and it will link the northwestern and eastern-southeastern sections of Beijing.

There are currently 4 metro lines serving the capital city that carry 1.5 million commuters everyday.

Transportation experts have been exploring other possibilities to use the city's underground to help ease traffic pressure.

At the "International Academic Conference on Underground Space" conference this weekend, the Beijing Urban Planning Commission and Beijing Urban Planning and Designing Research Institute jointly released a plan for the construction of six underground expressways by 2020.

Shi Xiaodong, a senior planner of the institute, said that moving traffic underground will help lessen noise pollution and ease heavy traffic in the old urban area.

A voice of caution was heard, however, when Duan Liren, an expert with the Beijing Transportation Management Engineering Institute warned that a large-scale underground expressway system would be unprecedented in the world and will have technological difficulties and construction costs that will exceed the metro system.