Researchers at Xidian University in Xi'an, Shaanxi province, have utilized long-range radar to inspect full cross-sections of 77 railway tunnels in eight hours, breaking the industry speed record.

The radar system, developed by Professor Su Tao's team at the university's Hangzhou Institute of Technology, is fitted to moving trains and can operate at up to 160 kilometers per hour, which is more than 30 times faster than conventional inspection methods. The team said the system offers new maintenance solutions for a major challenge faced by China's vast and expanding rail network.

Tunnels are critical nodes in the network. Over time, they can develop hidden problems such as internal voids, lining delamination and water ingress. If left undetected, these defects can pose safety risks.

According to the National Railway Administration, at the end of 2023, China had 17,177 operational railway tunnels, with a combined length of 22,669 km. High-speed rail accounted for 5,642 of those tunnels and 10,077 km of the total. With more lines opening in recent years, the numbers continue to rise.

Traditional inspection methods are still manual. Inspectors must enter dark tunnels during brief intervals between trains, tap tunnel walls and rely on experience to identify problems. "That approach is slow, risky and produces few traceable digital records," said project leader Xu Zhi.

"For a network that measures tens of thousands of kilometers, it makes routine inspection impractical. Real innovation must be rooted in field conditions."

The team moved its development work into active tunnels and maintained on-site presence for four years. They addressed practical problems, including how radar signals behave in arched tunnels, how to recognize signal patterns that indicate defects, and how to filter out noise caused by train vibrations.

The equipment can perform a full cross-section scan from more than 4.5 meters away from the tunnel wall, removing the need to crawl along the lining. Being train-mounted also enables the high scan speeds that made the 77-tunnel survey possible.

After detection, railway engineering experts drilled core samples at locations flagged by the radar for verification. Core analysis showed a high correspondence between the radar readings and physical evidence of thin lining and internal voids, producing accurate and directly verifiable diagnoses.

"High precision rapid inspection is only the first step; we must also make results immediately intelligible," Xu said, adding that the team is developing automated processing for large detection datasets and three-dimensional visual reconstruction.

"Soon, inspectors riding inside a train will be able to view an image of a tunnel's interior on a screen, with the location, type and scale of defects clearly visible. It will be like giving the tunnel a full body CT scan," he added.