The world's largest shield machine for railway construction, a tunneling device with a diameter of 15.4 meters, reached the middle point of the tunnel's underwater section on Wednesday, marking a milestone in the construction of the tunnel beneath the Yangtze River for a high-speed railway.

Such progress also means that the China-developed, 148-meter-long shield machine, is coming closer to the zone with the highest level of construction difficulty, said China Railway Tunnel Group Co, builder of what will be a 14.25-kilometer underwater railway tunnel linking Shanghai's Chongming Island with Taicang city, Jiangsu province.

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

"The shield machine called Linghang is currently working at 66 meters underneath the river and is expected to reach the deepest location — 89 meters underneath the river — in roughly four months. At that point, the water pressure is 0.9 megapascal, as overwhelming as six people standing on a fingernail," he said.

Progressing ahead at a speed of 24 to 28 meters per day, the shield machine that started working in April last year has excavated its 2,830th ring segment to reach 5,660 meters on Wednesday.

High-speed trains are expected to run at a maximum speed of 350 km/h inside the tunnel — the highest speed in underwater tunnels around the globe, without any compromise in speed compared with outside the tunnel.

This route will be part of the Shanghai-Nanjing-Hefei high-speed railway that is scheduled to be in operation by late 2029, according to China Railway Tunnel Group Co.