I'm a railfan.

I was reading something on the net about the layout of Stratford station
on the Central Line. I have been there before a few years ago, but I
don't remember the layout myself. The page indicated that it's possible
to transfer between Central Line trains and mainline trains across the
platform. Doesn't this imply that it's possible to board a tube train
without entering a fare-control area? If so, how does one get charged
the proper amount on one's farecard upon exit on the other end of one's
journey?

Peace... Sridhar