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Passengers board a rail transit train in southwest China's Chongqing, Dec. 28, 2018. North-east part of Chongqing rail transit loop line started trial operation on Friday. (Xinhua/Wang Quanchao)

Passengers board a rail transit train in southwest China's Chongqing, Dec. 28, 2018. North-east part of Chongqing rail transit loop line started trial operation on Friday. (Xinhua/Wang Quanchao)