asdf wrote:

Are there any cases where a travelcard holder can make a detour journey,
even an unusual one, staying entirely within validity but get charged an
extension because the system assumes a trip into zone 1?

Yes, this will happen in every such case, as the system does not know
which route the passenger actually took.

Each pair of origin/destination stations is assigned a set of zones
that the passenger is assumed to pass through when travelling between
them.

Which does somewhat clash with the basic concept of the travelcard...

For example, Finchley Road to Barons Court would be defined as a Z12
journey. Even if you travelled via Rayners Lane, a Z2345 journey, you
would still be charged for a Z12 journey (including excess fares as
appropriate). If you held a Z2345 Travelcard season and wished to
avoid the Z1 excess fare, you could leave and re-enter the station at
Rayners Lane, so that you would instead be making two separate Z2345
journeys.

So if someone did make the round about journey and got charged on exit - or
even had no money on their PAYG - what would happen if they pursued a
refund? As far as I can make out, they would have made a legitimate journey
using a totally valid means of travel and been charged additional. That
sounds like the basis of an interesting legal case.

Conversely, this also means that if your journey is defined as not
being via Z1, you can legally travel via Z1 and not get charged the
via-Z1 fare.

Well yes - but if you get checked at an interchange or en route (not that
TfL seems to bother with human ticket checks) would that line get you off
the hook?