Sparks fly as a rail mobile flash butt welding machine works on the track of the Guangzhou Metro Line 18 in Guangzhou, south China's Guangdong Province, on Dec. 16, 2020. A total of 122.6 kilometers of railway tracks will need to be laid for the Guangzhou Metro Line 18, a subway line which is under construction with a maximum designed speed of 160 kilometers per hour. (Xinhua/Liu Dawei)

Sparks fly as a rail mobile flash butt welding machine works on the track of the Guangzhou Metro Line 18 in Guangzhou, south China's Guangdong Province, on Dec. 16, 2020. A total of 122.6 kilometers of railway tracks will need to be laid for the Guangzhou Metro Line 18, a subway line which is under construction with a maximum designed speed of 160 kilometers per hour. (Xinhua/Liu Dawei)

Welding sparks are seen on the track of the Guangzhou Metro Line 18 in Guangzhou, south China's Guangdong Province, on Dec. 16, 2020. A total of 122.6 kilometers of railway tracks will need to be laid for the Guangzhou Metro Line 18, a subway line which is under construction with a maximum designed speed of 160 kilometers per hour. (Xinhua/Liu Dawei)

Workers work at the construction site of the Guangzhou Metro Line 18 in Guangzhou, south China's Guangdong Province, on Dec. 16, 2020. A total of 122.6 kilometers of railway tracks will need to be laid for the Guangzhou Metro Line 18, a subway line which is under construction with a maximum designed speed of 160 kilometers per hour. (Xinhua/Liu Dawei)

Workers work at the construction site of the Guangzhou Metro Line 18 in Guangzhou, south China's Guangdong Province, on Dec. 16, 2020. A total of 122.6 kilometers of railway tracks will need to be laid for the Guangzhou Metro Line 18, a subway line which is under construction with a maximum designed speed of 160 kilometers per hour. (Xinhua/Liu Dawei)

A worker measures the flatness of a welded joint on the track at the construction site of the Guangzhou Metro Line 18 in Guangzhou, south China's Guangdong Province, on Dec. 16, 2020. A total of 122.6 kilometers of railway tracks will need to be laid for the Guangzhou Metro Line 18, a subway line which is under construction with a maximum designed speed of 160 kilometers per hour. (Xinhua/Liu Dawei)