London Overground is an urban train service in London. Like the London Underground it is operated by Transport for London, though it remains part of the UK's National Rail network.
London Overground consists of five lines: the East London line (from Dalston Junction to West Croydon and New Cross); the Gospel Oak to Barking line; the North London line (from Richmond to Stratford); the West London line (from Willesden Junction to Clapham Junction); and the Watford DC line from London Euston to Watford Junction.
Brand new air-conditioned electric trains now operate on almost all London Overground services, and onboard WiFi is planned for the end of 2012. You can buy cheap train fares online from MyTrainTicket for all London Overground journeys, as well as London Travelcards (London Overground train travel is included as a transport option within the applicable Travelcard zones).