Paul Corfield wrote:

The answer to this is very simple. He might have a suspicion but he
could not prove it. There are a number of places where passengers can
enter the LU network from National Rail by crossing between platforms.
There are no gates. There are validators but are there signs to demand
that *everyone* with an Oyster card validates? - no there are not. There
*are* signs saying that if you use Pre-Pay on Oyster cards that you
should do so. There is NO requirement for a Travelcard user to do so
unless they are making a Pre-Pay trip as an extension to the Travelcard.
In such cases they are rather obviously using Pre-Pay!

Except it's not so obvious...

In my case, entering the system from National Rail at Forest Gate I pass
signs telling me "Oyster PrePay is NOT valid from this station". And there
is nothing at all at that station for me to touch in/out of, even for the
conscientious traveller who wants to help National Rail and TfL get a better
picture of service use.* I get to Stratford to change for the tube. I do not
think of myself as a PrePay user and that's all the sign tells me need to
touch in. So I have no need to touch in there. Then I get the tube.
Sometimes I come out through barriers, sometimes not - how does it tell?

And if I go out of zone, say to Amersham or Chesham, that's the first time
the ticket picks me up. And this brings up the other problem. The travelcard
is in part appealing because of "no need to plan your journey in advance",
whilst Oyster PrePay has been advertised (dodgily) on the same basis. But
the extensions *do* require me to beep in and out at Stratford, even if I
haven't yet planned to do so - say I was going to Harrow but got a phone
message to go to Chesham in the Willsden area?

Incidentally, if I'd started my journey at Ilford were there are barriers
with Oyster readers, would the system have a problem? Or would I need to
find a TfL reader for PrePay as well?

( * There used to be readers on the platforms but these were removed. And I
can't see how you can easily gate up (b-boom!) Forest Gate easily taking
into account people overshooting and wishing to double back, the frequent
switching of Ilford bound trains to platform 4 which is cannot be accessed
from the station building and requires stepping out onto street, and the
architecture of the station not really offering a good place for a row of
barriers for platforms 1-3.)

(Oh and yes I know that for Forest Gate to Metropolitan Line destinations
the best interchange is Liverpool Street, with two sets of barriers, but the
Circle/Hammer & Met central London sections are often being worked on at
weekends, making Stratford a better interchange.)