Technicians take part in the cold-running test at a section of Shanghai Metro Line 18 in east China's Shanghai, April 26, 2021. The cold-running test of the first phase of the northern section of Shanghai Metro Line 18, from Yuqiao to South Changjiang Road, began on Monday, and will be followed by a series of tests, including power transmission and hot-running tests.(Photo: Xinhua)

Technicians take part in the cold-running test at a section of Shanghai Metro Line 18 in east China's Shanghai, April 26, 2021. The cold-running test of the first phase of the northern section of Shanghai Metro Line 18, from Yuqiao to South Changjiang Road, began on Monday, and will be followed by a series of tests, including power transmission and hot-running tests.(Photo: Xinhua)

Technicians take part in the cold-running test at a section of Shanghai Metro Line 18 in east China's Shanghai, April 26, 2021. The cold-running test of the first phase of the northern section of Shanghai Metro Line 18, from Yuqiao to South Changjiang Road, began on Monday, and will be followed by a series of tests, including power transmission and hot-running tests.(Photo: Xinhua)

Technicians take part in the cold-running test at a section of Shanghai Metro Line 18 in east China's Shanghai, April 26, 2021. The cold-running test of the first phase of the northern section of Shanghai Metro Line 18, from Yuqiao to South Changjiang Road, began on Monday, and will be followed by a series of tests, including power transmission and hot-running tests.(Photo: Xinhua)

Technicians take part in the cold-running test at a section of Shanghai Metro Line 18 in east China's Shanghai, April 26, 2021. The cold-running test of the first phase of the northern section of Shanghai Metro Line 18, from Yuqiao to South Changjiang Road, began on Monday, and will be followed by a series of tests, including power transmission and hot-running tests.