Railway workers in Harbin, northeast China's Heilongjiang Province, are facing temperatures of minus 20 degrees Celsius during pre-dawn inspections of the tracks that are vital to keep the trains running safely. /Xinhua

The extreme cold at this time of year might cause subtle damage to the Harbin-Dalian high-speed railway. /Xinhua

The inspections are particularly important as China enters the Spring Festival travel rush, the largest annual mass migration of people on the planet. /Xinhua

Around a billion people will make over three and a half billion journeys in a 40-day period as they return home or travel to celebrate Chinese New Year. /Xinhua

The track checks have to be carried out during the railway network’s quiet time from shortly after midnight to 4:30 a.m., when Harbin is generally at its coldest. /Xinhua