The Beijing Subway, which began operating in 1969, now has 23 lines, 404 stations, and  of lines. It is the longest subway system in the world and first in annual ridership with 3.66 billion rides delivered in 2016. In 2013, with a flat fare of ¥2.00 (0.31 USD) per ride with unlimited transfers on all lines except the Airport Express, the subway was also the most affordable rapid transit system in China. The subway is undergoing rapid expansion and is expected to reach 30 lines, 450 stations,  in length by 2022. When fully implemented, 95% of residents inside the Fourth Ring Road will be able to walk to a station in 15 minutes. The Beijing Suburban Railway provides commuter rail service to outlying suburbs of the municipality.