BEIJING, Dec. 30 (Xinhua) -- The total length of operational subway lines in Beijing were extended to 637 km on Sunday with the trial operation of two extension lines.

The Line 6 extension and the third and fourth phase of Line 8, which cover 28.6 km, went into trial operation on Sunday, hoping to ease traffic congestion in the capital city.

Beijing now has 22 metro lines. In 2017, the city's metro system measured 608 kilometers. Shanghai has the country's longest metro systems with a total length of 705 km in operation as of Sunday.