transport to the International Arrivals Building as well as transport for Concourse D. They have all been given names based on the postal abbreviations of 50 states, e.g., VA, MD, AK. The Metropolitan Washington Airports Authority has gradually phased out the mobile lounge system for inter-terminal passenger movements in favor of the AeroTrain, an underground people mover which currently operates to all of the concourses except concourse D, with passenger tunnels remaining to concourses A and B. Plane mates remain in use to disembark international passengers and carry them to the International Arrivals Building, as well as to transport passengers to and from aircraft on the hard stands (i.e., those parked remotely on the apron without access to jet bridges). Main terminal Dulles's iconic main terminal houses ticketing on the upper level, baggage claim and U.S. Customs and Border Protection on the lower level, and annexes for the International Arrivals Building for international passenger processing, as well as the four Z gates (used by Air Canada and United Express), various information kiosks and other support facilities. The main terminal was recognized by the American Institute of Architects in 1966 for its design concept; its roof is a suspended catenary providing a wide enclosed area unimpeded by any columns. The main terminal was extended in 1996 to —Saarinen's original design length—which was slightly more than double its originally constructed length of . On September 22, 2009, an expansion to include the International Arrivals Building opened for customs and immigration processing with a capacity to process 2,400 passengers per hour. Also in September 2009, a central security checkpoint was added on a new security mezzanine level of the main terminal. This checkpoint replaced previous checkpoints which were located behind the ticketing areas, however, travelers enrolled in TSA PreCheck and CLEAR still use this area to clear security. A separate security checkpoint is available on the baggage claim level. Both security checkpoints connect to the AeroTrain, which links the main terminal with the A, B, and C concourses. Midfield terminals All airlines aside from Air Canada Express and United Express operate out of two linear satellite terminals. Each terminal is divided into two concourses, with the north terminal containing Concourses A and B, and the south terminal containing Concourses C and D. Concourses A and B Concourses A and B are located in the midfield terminal building closer to the main terminal. They