Me: Which train did you come on?
Dad: The overground
Me: Oh, I'd have come through London Bridge if I were you.
Dad: I did.
Me: Oh, so you did come on National Rail.
Dad: Yeah, that's what I said, the overground.
Me: (Give up).

On the way home:
Dad: Whcih way do you think I should go?
Me: I'd go on the Overground.
Dad: What, not go via Canada Water.
Me: Sigh.

The fact that much of the Underground goes overground makes it nonsense to call the Overground the Underground. To all intents and purposes and (crucially in my view) for ticketing, the Overground acts like a tube line and should be considered as such.