BEIJING - Beijing's subway system marked its 42nd year of operation on Tuesday, celebrating four decades of expansion that have made it the second-longest subway in the world.

Beijing's subway system is expected to carry approximately 10 million passengers daily in 2013, surpassing the world's busiest subway in Moscow, which transports 9 million passengers daily, according to Beijing's transportation department.

Built in the 1960s, the first stretch of Beijing's subway was initially used for national defense. The subway line began to allow civilian passengers on Jan 15, 1971.

The subway system now consists of 16 lines that transport 8.7 million passengers daily. The recent opening of four new lines has increased the length of the transit system to 442 km.

Jia Peng, spokesman for the Beijing Subway Operation Company, said the expansion of the network has called for an improved scheduling system, regular maintenance of subway equipment and improved personnel training.

The speed and density of the subway's construction are unparalleled in history, said Hao Weiya, vice general manager of Beijing Infrastructure Investment Co. Ltd.

Beijing's subway lines are expected to surpass 700 km in length by 2015, Hao said, adding that they will be built with an investment of 500 billion yuan.

China's urban transit has entered a period of rapid development, said Gao Yucai, director of the Urban Rail Transportation Commission under the China Communications and Transportation Association (CCTA).

Forty Chinese cities are expected to have their own subway networks by 2020, with a combined length of more than 7,000 km, Gao said.

Although the development of subway transit has brought a great deal of convenience to Chinese urbanites, accidents in recent years have led to calls for strengthened security and maintenance.

One boy died and 30 others were injured when a rising escalator collapsed on Beijing's Line 4 subway in July 2011.