北京地铁3条新线进入开通倒计时，年底北京城市轨道交通运营总里程将达909公里

Group 1 - The core point of the article is that three new subway lines in Beijing, specifically the middle section of Line 17, Line 18, and the southern extension of Line 6, are set to open by the end of the year [1] - Upon completion of these new lines, the total operational mileage of Beijing's urban rail transit will reach 909 kilometers by the end of the year [1]