MIG wrote:

It may also be geographical. Anywhere outside of northish London,
there would be no need to make the contrast.

I don't think I used the term myself, but I know that others did,
particularly where there was a choice, eg Stratford to Liverpool
Street, or Walthamstow Central into central London.

I only ever heard it in the phrase "Wood Green overground station" (now
called Alexandra Palace station) to avoid confusion with Wood Green
Underground station.