To combat congestion, runway 18/36 was added in the 1980s and the terminal was again expanded with the $250 million (or $58 million according to the "New York Times") 24 gate Concourse E opening in 1988, despite Denver's replacement airport already being under construction. By the early 1990s, Concourses A and B were exclusively used by United and United Express, Continental used most of Concourses C and D, and most other airlines moved to Concourse E. In the early 1990s, several charter services to the United Kingdom were introduced, and Martinair inaugurated services to Amsterdam up until Stapleton's closure. Continental closed its Stapleton pilot and flight attendant bases in October 1994, reducing operations and making United the airport's largest carrier.