A direct rail link between Beijing's two major railway stations has become operational, marking the opening of China's first underground rail line.

The 9.15-km-long line, which first opened in the early morning on Saturday, connects Beijing Railway Station and Beijing West Railway Station. It enables passengers to make a one-way trip between the two stations in about 15 minutes.

The line is expected to bring greater convenience to passengers who transfer trains between the two stations. In the past, they had to take the subway or bus to go to another station. The underground link will also speed up China Railway Corp's effort to connect the Beijing-Harbin and Beijing-Guangzhou rail lines, the two trunk lines stretching from the chilly northeastern regions to the warm southern provinces.

The maximum speed of trains running on the line is 12 km per hour, said Beijing Railway Bureau. Construction of the line started in December 2005 and was completed in July 2014. It took nearly nine years because the line goes through Beijing's dense subway network, which added technical complexity to the project, while Chinese engineers lacked experience building an underground railway, according to the bureau.

Zhao Jian, a professor of rail transportation at Beijing Jiaotong University, said the line will become very busy in the future because it can greatly extend the length of railways that travel through Beijing.