China’s metro network is expanding fast, as more and more second-tier cities open metro lines.

Over the past five years, 42 Chinese cities have been approved to build metro networks, and the number of cities with metro lines has increased to 27 from 17. Shijiangzhuang, Urumchi, Changchun and other second-tier cities are set to build their first metro lines in the near future.

The total length of subway lines across the country has reached 3,169 kilometers, and that number is estimated to surpass 6,000 kilometers by 2020, said Ren Hong, deputy director of the Department of Basic Industries under the National Development and Reform Commission.

Many Chinese commuters opt to travel by metro, which is fast, stable, reasonably priced and free from traffic congestion. Statistics show that the annual passenger volume of urban rail transit increased from 8.7 billion in 2012 to 16.09 billion in 2016.

Metro lines have also expanded urban space. Nanjing, capital city of eastern China’s Jiangsu province, is planning to build a metro line that connects the urban center with a distant suburb. According to the plan, the suburb will have nursing homes, residential areas, cultural and creative industries and more.

The city of Wuhan in central China is also working to open traffic in advance of further city development, which has already yielded positive results.