The longest tunnel for the Beijing-Zhangjiakou high-speed railway was dug through on Thursday, marking a major breakthrough in the 174-kilometer rail line which is planned to fully open to traffic in 2019 to support the commute for the 2022 Olympic Winter Games.

Starting from construction in April 2016, the 12-kilometer-long New Badaling tunnel, a pivotal part of the line, is also the country's most complicated underground burrowing project. Besides the drilling into the hard rock, the station beneath the Great Wall has 78 caverns and lots of intersections which create many difficulties in excavation. New techniques are being used to explore the tunnel carefully and avoid any damage to the Great Wall and the railway line designed and built 100 years ago.

The Beijing-Zhangjiakou high-speed railway is expected to be completed by the end of 2019, with travel time between Beijing and Zhangjiangkou reduced from three hours to just one hour.