Airports of London cannot be expanded due to the docks on either side. It is also the only airport serving London which does not operate at night. A light rail service from London City Airport DLR station, which adjoins the terminal building of London City Airport, links it to the financial district of the City of London at Bank and Monument stations, which offer interchanges with London Underground. Located in the London Borough of Hillingdon, Heathrow is by far the largest of London's airports and considered the main gateway into the United Kingdom for non-European visitors. Heathrow has four terminals and two parallel
London Underground departmental stock London Underground departmental stock Departmental locomotives on the London Underground consist of vehicles of a number of types which are used for engineering purposes. These include battery locomotives, diesel locomotives, electric locomotives, sleet locomotives, pilot motor cars and ballast motor cars. Details of the first four types are covered elsewhere. Pilot motor cars and ballast motor cars are generally vehicles which have been withdrawn from passenger service, but continue to be used by the engineering department. Pilot motor cars are used to move other vehicles around the system, while ballast motor cars are used to haul ballast trains and engineering