"Malcolm & Nika" writes:
So I went to Vauxhall and checked out what you said.
Sure enough there was a revenue exercise there yesterday.
You ask what it is that they can see that the gates dont?

They are solely checking for people who travel through zone 1
without a zone 1 ticket. This is marked on the ticket with the
nice colour triangles... If it was bought north of the river in
a zone 3 to a zone 2 station it will have north colourings.
It will still let you out in a zone 2 station allowing you to
travel through zone 1.

That makes sense -- except for the part about the gates.

If there are tickets that would be valid in terms of starting and
ending zones alone, but aren't valid for any possible route from the
point of issue to a particular station, why isn't it the case that
(1) the tickets carry enough encoded information for this to be
worked out, and (2) the gates are programmed to reject them?

Of course it gets trickier in cases where *is* a route within the
zones paid for, but it'd be longer than the shortcut through Zone 1,
because the passenger could claim to have taken the long route.
The gates could still detect this if the tickets carried information
about time of use, but then they'd have to know the fastest time
that the long route could take.
--
Mark Brader, Toronto | "Common sense isn't any more common on Usenet
| than it is anywhere else." --Henry Spencer

My text in this article is in the public domain.