Another big problem was the intense heat in the tunnel as temperatures regularly reached 40 C and the humidity remained steady at about 85 percent.

Ice cubes were transported to the tunnels without interruption to help cool down workers. However, they had to take turns working in the tunnel every three hours.

The 330-kilometer rail line, designed for trains running at speeds of up to 140 km per hour between Dali and Ruili, all in Yunnan province,will cut travel time to about three hours and 30 minutes from the current seven hours.