Washington Dulles International Airport and is on pace to exceed Ronald Reagan Washington National Airport's annual passenger numbers for 2018 after having fewer passengers ever since 2015. However, Dulles Airport still ranks behind Baltimore–Washington International Airport (BWI) in total annual passenger boardings, despite being a larger facility with more gates. Prior to World War II, Hoover Field was the main commercial airport serving Washington, on the site now occupied by The Pentagon and its parking lots. It was replaced by Washington National Airport in 1941, a short distance southeast. After the war, in 1948, the Civil Aeronautics Administration began to consider sites for a
Washington Dulles International Airport mezzanine with more room for lines. A new train system, dubbed AeroTrain and developed by Mitsubishi, began in 2010 to transport passengers between the concourses and the main terminal. The system, which uses rubber tires and travels along a fixed underground guideway, is similar to the people mover systems at Singapore Changi Airport, Hartsfield–Jackson Atlanta International Airport, and Denver International Airport. The train is intended to replace the mobile lounges, which many passengers found crowded and inconvenient. The initial phase includes the main terminal station, a permanent Concourse A station, a permanent Concourse B station, a permanent midfield concourse station