The 'railroad icemen' are a group of people who are vital for the safety of the railways. [Photo/Xinhua]

The line, which runs between Xi'an, in Shaanxi province and Zhengzhou, in Henan province, carries more than 300 trains a day.

The Guanyintang Tunnel was built in 1919 and has a length of 601 meters.

The tunnel has grown dilapidated with age, and the water that seeps through the cracks is condensed into ice in this high-altitude cave.

The ice could cause short-circuits, and pose risks to speeding train and passengers.

So, at 6:40 am the ice team arrive at the site.

Then, after checking their icebreaking tools they wait for the two-hour "blank time", which is typically from about 7:00 to about 9:00 am daily, when there is no train passing through Guanyintang Tunnel from the west to the east.

At 7:20 am the team enter the dark tunnel and meet with a damp, windy atmosphere.

At this point, the temperature inside the tunnel is - 10 C and a drop of water can turn into ice within moments.