China is on track to building what is eventually expected to be one of the largest transportation systems in the world as it ramps up its subway network.

Apart from Beijing and Shanghai, other major cities in the country already have their own subway systems or are in the process of building one.

In Changsha, the capital of Hunan province, workers take a walk along a tunnel of a subway construction site (left), as work on the city's railway network continues.

Changsha Metro was first opened on April 29, 2014, making Changsha city, the capital of Hunan province, the 19th city in mainland China to have a rail transit system.

According to the city's transport masterplan, the Changsha metro system will have six lines spanning 230km by 2020.

By 2030, a total of 12 main lines of urban rail transit will be built, covering a distance of about 456km.

The subway system is slated to have a total of 333 stations.