[msrpotus]

There are parts of the Underground that are, in fact, above ground and elevated (especially farther out like Ealing and Stratford). Then there’s also the Overground, which I think counts as light rail, but is generally on ground level.

[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