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 ticketing as 'auto top-up'. Tickets and pay as you go credit can be purchased via a website or over the telephone. The Oyster card system is designed to eliminate the need to purchase tickets at the station for most users. Following the implementation of the technology London Underground reduced the number of staff working in ticket offices. Contactless Visa, Maestro, MasterCard and American Express debit and credit bank cards issued in the UK, as well as contactless enabled smartphones and smartwatches using Apple Pay and Android Pay are accepted for travel on London Underground, London Overground, Docklands Light Rail, most National