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
Automation of the London Underground to open and close doors, and to assist in case of emergency. This method of working will be extended to the other lines by 2022 and will also be used on Thameslink and Crossrail. There are already driverless trains in operation on London second rapid transit system, the Docklands Light Railway (DLR), although this is a relatively newly built system. Boris Johnson promised in 2012 that there would be driverless Tube trains within 10 years. During his campaign to be re-elected as London Mayor, he said "TfL (Transport for London) will rapidly establish a timetable for introducing the first driverless