Note that in common parlance within London, the term "overground" may be used in reference to any National Rail line, in order to distinguish it from the Underground.

[3][4] A pilot scheme was launched on 30 September 2003 to bring National Rail services operated by multiple companies at 41 stations under one branding umbrella within London.

Railway stations selected for the Overground Network were those with a service frequency of 4 or more trains per hour to a London terminus.

[11] A diagrammatic map of the Overground Network was published to support passenger information provision, showing high-frequency routes in colour, with Underground and Tramlink connections.

Although this pilot was purely an exercise in branding, this was the first instance of TfL having a visible influence over National Rail services in London.

However, TfL has helped fund several similar station upgrade programmes including enhanced passenger information and line guides without applying the "on" branding, leaving the future of the scheme in doubt.

Again, according to Innes Ferguson, TfL's medium-term aspiration is for the former to be expanded to cover other National Rail services in London and the South-East, either by direct TfL operation through a concession-holder (as with the ex-Silverlink routes) or by a TOC agreeing to operate under the London Overground "brand".