China Completes First High-Speed Rail Tunnel Under Yangtze

China has completed its first high-speed rail tunnel beneath the Yangtze River, marking a major engineering milestone for the country’s national rail network. The 11.9-kilometer (7.4-mile) Caiyuanba Yangtze River Railway Tunnel in Chongqing was finalized on July 24, 2026, and will form a critical link on the Chongqing-Xiamen High-Speed Railway corridor, connecting the southwestern interior with the southeastern coast.

Context

The tunnel connects Yuzhong District and Nan’an District in the sprawling megacity of Chongqing, according to Xinhua News Agency. It is a key segment of the Yu-Xia (Chongqing-Xiamen) High-Speed Railway, itself a vital component of China’s “Eight Vertical and Eight Horizontal” national high-speed rail network — a system designed to crisscross the country with 16 major high-speed rail corridors stretching from Gansu in the northwest to Hong Kong and Shenzhen in the south.

The project was designed by China Railway Second Institute and constructed by China Railway 14th Bureau Group. It represents one of the most complex tunneling challenges ever undertaken in China, combining mountain tunneling, urban tunneling beneath a dense megacity, and riverbed excavation under the country’s longest river — all in a single project.

Key Developments

Construction of the tunnel presented extraordinary geological and urban obstacles. The route passed under 126 buildings, crossed metro lines six times, traversed the Yangtze riverbed, and cut through Nanshan Mountain — all while maintaining millimeter-level precision.

“The construction route passes under urban roads, elevated bridges, metro lines, multiple residential areas, the Yangtze River’s main channel, and Nanshan Mountain,” Yang Zhiyong, commander of the Wulong Command of Yu-Qian Railway Co., told Xinhua. “The overall construction difficulty is great and technical standards are stringent.”

The tunnel integrated three construction methods — open-cut, mining, and shield tunneling — making it China’s first large-diameter shield high-speed rail project to complete assembly, launch, reception, and disassembly of the boring machine entirely underground. The customized “Changjiang” (Yangtze) mud-water balance tunnel boring machine, with a diameter of 12.66 meters, was assembled in an innovative mushroom-shaped chamber 150 meters beneath Nanshan Mountain.

According to the South China Morning Post, the machine was designed to withstand high water pressure while cutting through alternating rock layers deep under the riverbed. The total shield-driven section extended 3,845 meters, with 1,282 meters directly beneath the Yangtze at water pressures reaching 0.93 MPa.

“The 11.9km Caiyuanba Yangtze River Railway Tunnel links two districts in Chongqing and will be a key link in a high-speed line between two key hubs in the southwestern megacity and Xiamen in the southeastern province of Fujian,” wrote Alice Li of the South China Morning Post.

Gang Shenfu, project manager from China Railway 14th Bureau Group, highlighted the tunnel’s technical achievements: the shield achieved high-precision settlement control while navigating through alternating layers of mudstone and sandstone beneath the riverbed.

Analysis

The tunnel’s completion removes a critical bottleneck on the Chongqing-Xiamen corridor. Sections from Chongqing East to Changsha and from Ganzhou to Xiamen are already in service, meaning the corridor is largely operational. Once the remaining Chongqing–Chongqing East and Changsha–Ganzhou segments are completed, a continuous high-speed rail link will connect Chongqing — home to approximately 32 million people — with the coastal port city of Xiamen in Fujian province.

The project advances China’s broader national infrastructure strategy. Under current plans, Beijing aims to “substantially complete” the 16-corridor high-speed rail network by 2030. As of March 2026, construction was 84 percent complete, according to state-owned China Railway Group, as reported by SCMP. The network already stretches across some of the world’s most difficult terrain, linking remote provinces with economic hubs.

Beyond its transport benefits, the tunnel represents a showcase for Chinese engineering prowess. The innovative underground assembly of the giant TBM and the coordination of three distinct tunneling methods demonstrate capabilities that could be applied to future infrastructure projects both domestically and abroad.

What’s Next

With the tunnel breakthrough achieved, attention now shifts to the remaining gaps on the Chongqing-Xiamen corridor. No specific completion timeline has been announced for the Chongqing–Chongqing East link or the longer Changsha–Ganzhou section. However, the steady progress toward the 2030 network completion target suggests continued momentum.

The Caiyuanba tunnel is unlikely to be the last tunnel under the Yangtze for high-speed rail. Reports indicate another project — on the Yancheng–Yixing line — is already using a 14.33-meter TBM, suggesting multiple Yangtze crossings will serve different high-speed corridors as China pushes toward its goal of one of the most extensive and interconnected high-speed rail systems in the world.