The London Overground name is the brand , which is used by the Transport for London ( TfL ) for train services on four railway lines in the London area : the Euston-Watford DC Line , the North London Line , the West London Line and the Gospel Oak to Barking Line .
It has been operated by London Overground Rail Operations since 2007 as part of the National Rail network , under the franchise control and branding of Transport for London ( TfL ) . It consists of five lines : the East London Line ; the Gospel Oak to Barking Line ; the North London Line ; the West London Line and the Watford DC Line .