The London Underground is a public rapid transit system in the United Kingdom that serves a large part of Greater London and adjacent parts of the home counties of Essex, Hertfordshire and Buckinghamshire. It has many closed stations, while other stations were planned but never opened for public use. Some stations were closed down because a scarcity of passengers made them uneconomic; some became redundant after lines were re-routed or replacements were built; and others are no longer served by the Underground but remain open to National Rail main line services. Many stations were planned as parts of new lines or extensions to existing ones but were later abandoned.