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　　On May 19, the main line of Dalian Metro Line 5 saw its two-way track-laying project completed by China Railway First Group Co., Ltd.

　　The undersea shield tunnel of Dalian Metro Line 5 is the world’s first large-diameter shield tunnel built through karst strata in a sea-area. With a total length of 2,882 meters, the project was regarded as an impossible task by top-notch construction experts. The section passing sea area measures 2,310 meters in length, with the lowest part of tracks in the undersea tunnel at 50 meters below the sea level. Altogether, 5,764 meters of the two-way rails were laid.