Most people search for a shady spot to avoid direct exposure to the mid-June sun that pushes temperatures to 39 C in Hekou Yao autonomous county bordering Vietnam.

However, Xie Yunchuan, 29, found the searing heat far more bearable than that in his place of work, the Taiyangzhai railway tunnel, where the temperature often rises to 43 C.

Workers in the Taiyangzhai tunnel, one of six tunnels longer than 5 km, on a key section of the railway connecting Hekou and Mengzi in Yunnan province. Photo by Feng Yongbin / China Daily

The trees glowed golden in the sunshine and the cicadas whirred to "express their antipathy to the hot weather", as a local saying goes. Feeling in need of refreshment, Xie hopped onto his motorcycle and headed off for lunch at an air-conditioned house nearby.

Inside the tunnel, air barely circulates, and Xie and his co-workers adopt measures to avoid suffocation and heatstroke, taking a break every two or three hours while another team continues the work.

"In many tunnels I've worked in, it's usually been quite cold inside. This is the first time I've encountered such high temperatures," he said.

The conditions mean the workers perspire more than usual. To avoid dehydration, Xie and his fellow workers have figured out how to make a refreshing, and possibly lifesaving, drink. They mix sugar, salt and tea and then place the concoction in the fridge before starting work.

Meanwhile, their company, the Fifth Bureau Group Corporation of China Railway, has spent 200,000 yuan ($32,600) on construction of an icehouse close to the entrance of the tunnel.

During every shift, six metric tons of ice are carried inside to lower the temperature in the tunnel, which will be 7,354 meters in length when completed. Despite working for four years, the team still has another 280 meters to drill before the tunnel will be finished, according to Jiang Jun, deputy head of the construction team.

The Taiyangzhai tunnel is a key part of the Mengzi-Hekou Railway, connecting Hekou Yao autonomous county and Mengzi, the capital of Honghe Hani and Yi autonomous prefecture in Yunnan province.

Upon completion next year, the 468-kilometer-long railway will reduce the current eight hour journey between Kunming, the provincial capital, and Hekou by half, according to Kunming Railway Bureau.

In the bigger picture, the Mengzi-Hekou Railway is the last component of a project that constitutes the eastern line of the Pan-Asia Railway, a crucial link between Southwest China and several Southeast-Asian countries.