Comment by tialaramex

> Oyster came in as travelcard only

I don't think this can be correct. One of the early Oyster "problems" was that people who'd routinely made some particular journey on their travelcard, perhaps for years, would get locked out of Oyster and be unable to start a second journey. The reason was that their Travelcard had never been valid for their journey but it was valid for both the entry station and exit station and before Oyster it had no way to connect the dots, now Oyster could see the journey route wasn't allowed and so it charged them, making their balance negative, whereupon they can't start a second journey until they pay for the first.

Edited to add: Oh wait, maybe you meant only on (what would be) Overground stations ? That seems very weird, but I guess I can't prove you're wrong.