China's first metro line built across under the Yellow River started trial operation on Sunday in Lanzhou, capital of northwest China's Gansu Province.

The city is located on the upper reaches of the country's "mother river" and its second longest river, which runs 5,464 km from west to east before entering the Pacific.

The fast urban transit has been a dream for the three million residents in the city, as the river flows through the urban area, creating a heavy traffic pressure on the road bridge. Construction of the metro line started in 2014.

The 25.9-km long section in trial service on Sunday is the first phase of Lanzhou Metro Line 1, passing through four of the city's urban districts from east to west.

The tunnel of the metro line passes through a unstable pebble layer, which is 200 to 300 meters deep, and highly permeable.

"The metro construction has accumulated experiences for other under-river urban rail and tunnel construction in similar geological and stratigraphic conditions," said Duan Tingzhi, chairman of the Lanzhou Rail Transit.

Huang Jianwei, deputy secretary-general of the China Urban Rail Transit Association, said that by the end of 2018, a total of 35 cities in China had opened 185 urban rail transit lines with an operation mileage of 5,761 km.

(Cover: China's first metro line built across under the Yellow River in Lanzhou, northwest China's Gansu Province, June 21, 2019. /VCG Photo)