Passengers pose for photos with the bullet train at the Mudanjiang Railway Station in Mudanjiang, northeast China's Heilongjiang Province, Dec. 25, 2018. A high-speed railway line officially went into operation on Tuesday, linking cities of Harbin and Mudanjiang in China's northernmost province of Heilongjiang, where the temperatures dip as low as minus 40 degrees Celsius. (Xinhua/Zhang Chunxiang)