BEIJING, Nov. 22 (Xinhua) -- The No.17 metro line in Beijing, the longest rail line under construction in Beijing, will be operational in 2021, municipal authorities said Wednesday.

The line, at 49.7 km, starts from Beijing Future Science Park in Changping and goes down to the southeastern part of the city in Yizhuang. It has 20 stops and designed speed of 100 km per hour, according to the Beijing Major Projects Construction Headquarters Office.

The line, which runs through the districts of Changping, Chaoyang, Dongcheng and Tongzhou, will take pressure off existing metro lines, the office said.

Relocation of residents is underway, the office said.