The world's largest shield machine for railway construction reached the halfway point of its underwater journey beneath the Yangtze River on Wednesday, marking a major milestone for the high-speed rail tunnel linking Shanghai's Chongming Island with Taicang, Jiangsu province.

Developed in China, the 148-meter-long machine — with a diameter of 15.4 meters — is edging closer to the most technically challenging section of the tunnel, according to China Railway Tunnel Group Co, the project's contractor.

"With a gradual downward slope, the pressure from the surrounding water on the construction project is rising," said Wang Yi, the project's deputy equipment manager.

Nicknamed Linghang, or "Navigator," the shield machine is currently operating 66 meters below the riverbed and is expected to reach its deepest point — 89 meters underground — in about four months, Wang said.

"At that point, the water pressure will be 0.9 megapascal — as overwhelming as six people standing on a fingernail," he said.

Advancing at a daily pace of 24 to 28 meters, the machine began operations in April last year and had completed 2,830 ring segments by Wednesday, extending 5,660 meters into the riverbed.

The 14.25-kilometer tunnel is a critical part of the Shanghai-Nanjing-Hefei high-speed railway, which is scheduled to open by late 2029, the company said.

Trains traveling through the tunnel are expected to reach a top speed of 350 kilometers per hour — the highest operating speed for any underwater tunnel in the world — with no reduction in speed compared to surface travel.

Engineers said the record-breaking project poses immense challenges to the shield machine, especially due to the tunnel's length, which requires the equipment to function far beyond conventional limits.

"To meet the demand, we use a main bearing with a service life of 16,700 hours, nearly triple the theoretical lifespan of similar components," Wang said. "That allows the machine to complete more than 15 kilometers of excavation in a single task."

The shield machine is also equipped with a proprietary "intelligent tunneling brain" system, designed to overcome obstacles such as ultra-large diameter drilling, complex geological conditions, ecological sensitivities and varying strata permeability.

"This project became the world's first to achieve unmanned tunneling operations, with staff required only to be on duty," Wang said.

The AI system integrates nine intelligent functions, including perception, tunneling, prefabrication, installation, detection and management, and is capable of making and executing operational decisions automatically, he said.

The tunnel must traverse 21 identified risk zones, and has already passed six, including a highway, a dock and a protected aquatic farming area, without incident, said Fu Bowei, deputy chief engineer for the project.

The Shanghai-Nanjing-Hefei line forms the eastern section of the larger Shanghai-Chongqing-Chengdu high-speed railway and is a key component of China's national rail network.

Once completed, it is expected to significantly reduce travel times between key urban centers in the Yangtze River Delta region — linking Shanghai with city clusters around Nanjing and Hefei — and contribute to the coordinated development of the Yangtze River Economic Belt and the integrated growth of the delta region.