I read so many different messages on this groups about what happens it you
do/don't touch in/touch out with Oyster and the issue of 'unresolved
journeys' (and if they are necessarily a problem) that I am thoroughly tied
in a knot. So in order for me to give someone sound advice...

On a commute between East Croydon and City Thameslink using an Oyster with a
period travelcard covering the whole journey, the gateline at East Croydon
deals with the touch-in and touch-out at that end. At City Thameslink there
are no gatelines but are Oyster validators (I don't know if they are marked
for Pre-Pay only but I've certainly not seen them used much when I've passed
through there). Is touching in and out at City Thameslink

a) always necessary
b) necessary sometimes (perhaps if you go on to enter a nearby tube station)
c) merely encouraged
d) of no benefit whatsoever
e) any other answer

Thanks. I suspect the answer is unlikely to be a).

I think I have been particularly bamboozled by having passed through
Wimbledon station lately. There _all_ Oyster users arriving on Tramlink are
warned to validate at their originating tramstop in order to be able to exit
at Wimbledon (though this advice no longer appears on 'Ask Oyster', just the
website) and I've never figured out why. I can understand the need for an
entry to the system to pair up with an exit but I would have thought it
would be possible for National Rail users to also have arrived without
having validated at their origin.

G.