The Capital Airport Express of the Beijing Subway, also known by the initials ABC, Airport Beijing City, is an airport rail link from Beixinqiao station to the Beijing Capital International Airport. The line became operational on July 19, 2008. On subway maps, the Capital Airport Express' color is purplish gray .

The Beijing Subway is the rapid transit system of Beijing Municipality that consists of 27 lines including 22 rapid transit lines, two airport rail links, one maglev line and 2 light rail lines, and 490 stations. The rail network extends 836 km (519 mi) across 12 urban and suburban districts of Beijing and into one district of Langfang in neighboring Hebei province. With 3.8484 billion trips delivered in 2018, an average of 10.544 million trips per day, the Beijing Subway is the world's busiest metro system. Single-day ridership set a record of 13.7538 million on July 12, 2019. Being 836 kilometres (519 mi) in system length, the Beijing Subway is the longest metro system by route length, surpassing the Shanghai Metro in December 2023.