BEIJING -- The main structure of metro line 12, which will run east and west through northern Beijing, will be finished by 2019, the Beijing Municipal Commission of Transport said on Saturday.

The line, with a total length of 29.6 kilometers, will have 21 stops, 15 of which are transfer stations, the commission said. About 60 percent of construction work has begun, and the main structure will be finished by the end of 2019, the commission said.

The line is expected to become operational in 2021. It will start at Sijiqing Bridge near the west fourth ring road and end in the Dongba area near the east fifth ring road in Chaoyang district.

Beijing has taken measures to ease traffic congestion in the city by building subway lines.

By the end of 2017, the city's metro system measured 608 kilometers. By the end of 2018, the system will be extended to 630 kilometers. Beijing now has 22 metro lines.