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
History of the London Underground changed from serving a closed loop around the centre of London on the north side of the River Thames to a spiral serving Hammersmith. On the Jubilee line a seventh car was added to all trains, in 2006 and a new signalling system allowing automatic operation was commissioned in 2011. On the Victoria line, new trains were introduced between July 2009 and June 2011, and a new signalling system has allowed 33 trains per hour from January 2013. The sub-surface (Metropolitan, District, Circle and Hammersmith & City) lines have been upgraded, with new air-conditioned S Stock. Also, the sub-surface track,