[rahimnathwani]

> presumably in most cases without realising

I doubt this. Even as a child I knew that a travelcard must be valid for all the zones you travel through. Of course, if challenged, someone knowingly cheating would claim they didn't realise.

> you tap pink validators at key interchanges > you'd need to pass through to do your slower > and less central route avoiding Zone 1"

Wow! I had no idea this was a thing. I moved away from London in 2010, and now I'm curious to know whether this was implemented before or after I left.

[Symbiote]

It was introduced on 6 September 2009: https://tfl.gov.uk/info-for/media/press-releases/2009/septem...

I used this frequently at some point, but it is now so long ago I can't remember where. Especially with many London Overground routes, it's not necessarily slower.