An attendant shows an oxygen cylinder on the train Z722 from Changchun of northeast China's Jilin Province to Lhasa of southwest China's Tibet Autonomous Region, June 16, 2016. The Z722 and Z21 trains from Changchun to Lhasa are equipped with oxygen supply equipments for passengers. They used 25T air conditioner train and started to operate on Feb. 26. (Xinhua/Wang Haofei)

Photo taken on June 16, 2016 shows an oxygen outlet above pillows on the train Z722 from Changchun of northeast China's Jilin Province to Lhasa of southwest China's Tibet Autonomous Region. The Z722 and Z21 trains from Changchun to Lhasa are equipped with oxygen supply equipments for passengers. They used 25T air conditioner train and started to operate on Feb. 26. (Xinhua/Wang Haofei)

Photo taken on June 16, 2016 shows the air conditioner equipment supplying oxygen on the train Z722 from Changchun of northeast China's Jilin Province to Lhasa of southwest China's Tibet Autonomous Region. The Z722 and Z21 trains from Changchun to Lhasa are equipped with oxygen supply equipments for passengers. They used 25T air conditioner train and started to operate on Feb. 26. (Xinhua/Wang Haofei)

Photo taken on June 16, 2016 shows an oxygen outlet on the train Z722 from Changchun of northeast China's Jilin Province to Lhasa of southwest China's Tibet Autonomous Region. The Z722 and Z21 trains from Changchun to Lhasa are equipped with oxygen supply equipments for passengers. They used 25T air conditioner train and started to operate on Feb. 26. (Xinhua/Wang Haofei)