The China-Laos railway reached a key milestone after engineers successfully drilled through its 70th tunnel on Monday in southwest China's Yunnan Province, wrapping up a four-year construction of the key section.

The tunnel's completion has laid a solid foundation for a punctual operation of the railway, which connects southwest China city Yunnan with the Laotian capital of Vientiane.

Located in the Mojiang Hani Autonomous County, the 11.84km-long tunnel runs through a mountainous area with treacherous geographical conditions.

Due to the high temperatures on the construction site, large air-conditioners and ice makers are being used to try and lower the temperature in the tunnel as much as possible and create a relatively good working condition for the workers.

"The rock temperatures in the Shitouzhai tunnel are roughly between 36 and 37 degrees Celsius. However, the temperature of our construction site can reach up to 50 degrees Celsius. To speed up the construction schedule, we sent extra shifts of workers and purchased large air-conditioners and ice makers to lower the on-site temperatures by 3 to 5 degrees Celsius," said Wang Li, manager of project department in Yuxi-Mohan Railway with China Railway 11 Bureau Group Co.,Ltd.

With a total length of over 1,000 km, the China-Laos railway will be put into operation by the end of 2021, along with the Boten-Vientiane Railway in Laos. After the train service is launched, traveling from Kunming to Vientiane will only take an one-day trip.