Workers build the New Badaling tunnel of the Beijing-Zhangjiakou high-speed rail line in Beijing, capital of China, Dec. 13, 2018. Workers have achieved a major breakthrough in the Beijing-Zhangjiakou high-speed rail line project, after they dug through the New Badaling tunnel, a pivotal part along the line, on Thursday. (Xinhua/Xing Guangli)

Workers celebrate as they cut through the New Badaling tunnel of the Beijing-Zhangjiakou high-speed rail line in Beijing, capital of China, Dec. 13, 2018. Workers have achieved a major breakthrough in the Beijing-Zhangjiakou high-speed rail line project, after they dug through the New Badaling tunnel, a pivotal part along the line, on Thursday. (Xinhua/Xing Guangli)

Workers celebrate as they cut through the New Badaling tunnel of the Beijing-Zhangjiakou high-speed rail line in Beijing, capital of China, Dec. 13, 2018. Workers have achieved a major breakthrough in the Beijing-Zhangjiakou high-speed rail line project, after they dug through the New Badaling tunnel, a pivotal part along the line, on Thursday. (Xinhua/Xing Guangli)