BEIJING, Dec. 14 (Xinhua) -- On a typical weekday in December, away from the rush hour crowds, a batch of aviation documents was loaded into the luggage compartment of the Beijing Daxing Airport subway line. In the passenger cabin ahead, a few commuters quietly sat, scrolling through their phones.
The Daxing Airport subway line, China's fastest urban subway system, runs at a top speed of 160 km per hour. It conn
