Because London's transport is all integrated under control of the local government (an option which is prohibited in most UK cities to facilitate "competition" ie price-gouging by foreign owned corporations) you might not have realised that you weren't using the Underground but one of the other modes, almost all of which are covered by the same ticketing system (the "Dangleway" isn't and neither are the boats).

If you travelled on ordinary seeming trains above ground, those weren't the Underground but instead ordinary "heavy rail" trains on National Rail infrastructure within London. Many of those lines are indeed far above the ground in parts of central London, but those are just called viaducts and aren't usually considered an "elevated railway" per se.

If you travelled on small trains in East London that didn't have a driver (you can sit at the front like a roller coaster), that's the Docklands Light Railway, almost all of which is elevated, it was built towards the end of last century during regeneration of the "Docklands" area.

If the trains seemed to be sharing a road with cars at street level, in South London, that's the Tramlink.

If the trains were smaller and red, that's a bus.