Line 10 of the Beijing Subway is the second loop line in Beijing's rapid transit network as well as the longest and most widely used line. A circle route (also circumference, loop, ring route, ring line, or orbital line) is a public transport route following a path approximating a circle or at least a closed curve.
The line is 57.1 km in length and runs entirely underground through Haidian, Chaoyang, and Fengtai Districts, either directly underneath or just beyond the 3rd Ring Road. The Line 10 loop is situated between two and six kilometers outside the Line 2 loop, which circumnavigates Beijing's old Inner City. Every subway line through the city center intersects with Line 10, which has 24 transfer stations along the route, and 45 stations in all. Line 10's color is Capri.
Line 10 is the world's longest subway loop line and one of the longest entirely underground subway lines requiring 104 minutes to complete one full journey in either direction.
The Beijing Subway network was originally conceived to have only one ring line. The booming economy and explosive population growth of Beijing put a huge demand on Line 2, surpassing its designed capacity. In 2001 and 2002, the China Academy of Urban Planning and Design proposed two "L-shaped" lines named Line 10 and 11. Together they would form a second loop around Beijing and relieve pressure on line 2.
According to Wikipedia