In message , Paul Corfield
writes
It isn't a matter of only acknowledging interchanges - it is about
knowing the fare between origin and destination. Origin and destination
is what will be checked with maybe a sanity check that expected
interchanges have been travelled through as expected within a journey
time parameter.
oh national rail these sometimes fail a sanity check however

Please note that I am speculating as to the method of
working here - I have not seen anything on how London Wide PAYG on all
rail modes will work.
I'm curious as to whether Gold Card discounts on national rail
extensions to season tickets will be supported on Oyster. No technical
reason why they couldn't be (expect for fiscal expense of getting the
programming right).

--
Paul G
Typing from Barking