[tialaramex]

The Overground is just a (branded) part of the normal National Rail heavy rail network that serves London. Same track, same stations, same rolling stock, just operated by TFL (Transport for London) like the rest of London's public transport.

[spiralx]

TFL control two heavy rail networks - the Overground and TfL Rail - although the latter will become part of the Elizabeth Line when it opens.

https://en.wikipedia.org/wiki/TfL_Rail