I'd also like to know the answer to this as I think it's hopelessly
unclear (as illustrated in the recent thread about Tramlink at
Wimbledon!) One interesting aspect that I've considered - at City
Thameslink and Blackfriars there are now frequent 'swoops' by
Thameslink ticket inspectors in the morning peak. These guys don't have
handheld Oyster readers - instead they use the Prepay validators to
check people with seasons on Oyster. Of course, as long as you have
prepay balance, an Oyster will *always* be accepted by these validators
(as it assumes you're entering the system, not leaving it). So far it
doesn't seem to have led to any unresolved journeys, but it doesn't
seem to be the 'right' way to check Oyster tickets. Perhaps Thameslink
staff need some training...

I suppose it does a job in that it makes no difference to anyone with valid
travelcards and those entering the station and with sufficient Pre-Pay, and
for those leaving the station the validators would actually say whether they
have registered an entry or an exit on Pre-Pay. At least I assume they do -
the ones on the tube at Wimbledon and Fulham Broadway do. So if they see
one display a message for entering the system they know the passenger had
never touched before the journey. Mind you after being nailed for a penalty
fare or worse the passenger would be left with an Oystercard that presumably
will have a potential unresolved journey on it. Tough titty really but not
the most elegant solution.