DW downunder reply@newsgroup wrote

That's something that has developed since 1987, when I returned to
Oz.
Certainly, none of US called anything the overground. The whole
concept of
Network South East was to give the whole home counties and beyond a
product
concept built on the Underground model of clarity (if not frequency).

If calling everything local on rails that's not LU "overground"
developed in
a generation, by 2027, all will be clear again!

The OED has a quote from 1931 about a bus line (Hadley Wood/ Barnet to
Victoria) which was branded as Overground !

--
Mike D