November 15, 2022, the airport's Washington Metro station opened as part of the Phase 2 extension of the Silver Line, from Wiehle–Reston East station to Ashburn. Planned development By the 1980s the original design, featuring mobile lounges to meet each plane, was no longer well-suited to Dulles' role as a hub airport. Instead, midfield concourses were constructed to allow passengers to walk between connecting flights without visiting the main terminal. Mobile lounges were still used for international flights and to transport passengers between the midfield concourses and the main terminal; Concourse C/D was the first to be built, followed by Concourse A/B. A tunnel (consisting of a passenger walkway and moving sidewalks) which links the main terminal and Concourse B was opened in 2004. The Metropolitan Washington Airports Authority (MWAA) began a renovation program for the airport including a new security 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 (with access to the current temporary C concourse via a tunnel with moving walkways), and a maintenance facility. Mobile lounges continue to service Concourse D from both the main terminal and Concourse A. Even after AeroTrain is built out and the replacement Concourses C and D are built, the mobile lounges and plane mates will still continue to be used, to transport international arriving passengers to the International Arrivals Building, as well as transport passengers to aircraft parked on hardstands without direct access to jet bridges. Dulles has stated that the wait time for a train does not exceed four minutes, compared to the average 15-minute wait and travel time for mobile lounges. Under the development plan, future phases would see the addition of several new midfield concourses and a new south terminal. A fourth runway (parallel to the existing runways 1 and 19 L&R) opened in 2008, and development plans include a fifth runway to parallel the existing