Staff of the China Railway Guangzhou Engineering Group (CRECGZ) pose for a group photo at the entrance to the Ban Nakok Tunnel in northern Laos' Oudomxay Province, on June 21, 2020. China Railway Guangzhou Engineering Group (CRECGZ) drilled through the Ban Nakok Tunnel, the second longest tunnel along the China-Laos railway, on Sunday. (CRECGZ/Handout via Xinhua)

File photo taken on Oct. 8, 2017 shows staff members of China Railway Guangzhou Engineering Group (CRECGZ) working in the Ban Nakok Tunnel in northern Laos' Oudomxay Province. China Railway Guangzhou Engineering Group (CRECGZ) drilled through the Ban Nakok Tunnel, the second longest tunnel along the China-Laos railway, on Sunday. (CRECGZ/Handout via Xinhua)

File photo taken on Oct. 12, 2017 shows the construction site of the Ban Nakok Tunnel in northern Laos' Oudomxay Province. China Railway Guangzhou Engineering Group (CRECGZ) drilled through the Ban Nakok Tunnel, the second longest tunnel along the China-Laos railway, on Sunday. (CRECGZ/Handout via Xinhua)

A staff member of China Railway Guangzhou Engineering Group (CRECGZ) works in the Ban Nakok Tunnel in northern Laos' Oudomxay Province, on May 28, 2020. China Railway Guangzhou Engineering Group (CRECGZ) drilled through the Ban Nakok Tunnel, the second longest tunnel along the China-Laos railway, on Sunday. (CRECGZ/Handout via Xinhua)