"Arthur Figgis" wrote

People have been calling National Rail trains in London "overground" for a
lot longer than LOROL has existed. There was even an Overground Network
branding on a few routes, which lasted about as long as it took for people
to say "WTF are the new signs about?" - but many of the signs are still
there.

I suspect people started referring to the overground at least as long ago as
the time the Wombles emerged from Wimbledon Common. Actually, Wimbledon is a
good place to make the distinction between the underground route via Earls
Court and the other routes into Waterloo or Blackfriars.
http://www.toonhound.com/wombles.htm

Peter