CHENGDU, July 31 (Xinhua) -- The first driverless subway line in the city of Chengdu, capital of southwest China's Sichuan Province, is under construction and the tunnel was dug through on Tuesday, according to the subway constructors.

The line, named the No. 9 Line of Chengdu, is a project co-built by Chengdu Rail Transit Group Co., Ltd. and China Railway Group Ltd.

Construction of the subway line started at the end of 2016, and it will become the first fully-automatic driverless subway line in west China.

The phase-I project runs approximately 22 km, with a designed top speed of 140 km per hour and 13 stations.

The subway line is scheduled to be operational by the end of 2020.