On a chilly spring day, men in bright yellow vests walk slowly and carefully along a railway track in Kaili, Guizhou province. Their eyes focused on tiny details that could mean life or death for the occupants of a passing train. In the vast, silent mountains, their vests add a speck of color as they carry out their painstaking inspection.

Zhou Jie is the railway technician in charge for China Railway Chengdu Group Corp. He works with 11 other inspectors on a section of the Shanghai-Kunming High-Speed Railway between Huangsi township and Longli county.

They describe themselves as "railway guards" whose job is to maintain the safety of the line.

It has been more than seven years since Zhou started railway inspection work. Every year, his team checks more than 2,000 kilometers of track, repairs more than 1,000 switches and eliminates scores of potential dangers. They cannot make mistakes, as doing so could be catastrophic.

"Get off the track, train's ahead!" Zhou calls loudly to alert his crew after receiving a message that a train was coming. Instantly, four crew members move two heavy steel-inspection devices away to safety.

After the Lunar New Year, the Shanghai-Kunming line was busy transporting people back to work.

"We routinely conduct checks between every two trains," Zhou said. "Our time includes walking on and off the track 60 times, stooping down hundreds of times to analyze over 100 waveforms to discover any rail deformations."

The physical demands are significant — walking more than eight hours a day carrying about 50 kilograms of detectors to inspect around 10 kilometers of railway line.

On winding railway sections with limited visibility ahead, Zhou makes a point of reminding his colleagues to concentrate on the work of accurate detection while maintaining their safety awareness.

Zhou works efficiently. He can quickly take up a detector and then continue his steady march ahead on the winding mountain track.

After identifying a spot on the rails with potential damage, Zhou squats to measure and check it with special tools. Then he marks the spot. He points to the detector.

"Track inspectors should pay extra attention in case of trouble under different circumstances. The real test is to find defects in millimeters every day among thousands of meters of track. If a little defect omitted, an accident can happen," he said.

Wang Jin contributed to this story.