On Tue, 22 Jul 2008 07:05:53 -0700 (PDT), google@woodall.me.uk wrote:

>What would be really neat, (but almost certainly not possible using a
>standard oyster card) would be to have "magic" cards that change their
>number.
>
>For example, a Sunday trip from Watford Junction to London with enough
>zone 1 travel to pass the z1-2 cap is cheaper with two cards. - 3.00
>each way from WJ->Euston plus 4-80 z1-2 cap. (Z1-8+WatfordJ cap is
>12.60) In theory it's maybe possible for the card to tell where it's
>being touched in or out before it reveals its serial number (at the
>very least it could possibly start a corrupted transmission first
>time). So rather than having to have two cards and remember which one
>to use when, the card could handle all that logic for you.
>
>(You can do even better if you touch out/in at willesden junction -
>total journey cost 6.80 - but that requires you to take the slow
>train. I can't see how any hack is going to be able to generate a
>valid touch out. I can that a faked touch in might be possible.)

What on earth would be the point in such an elaborate scheme? If
you've hacked the card then you can just add £100 (or whatever) of
PAYG credit, for free, whenever you feel like it.

Your card would probably be blacklisted during the nightly
synchronisation of the readers with the central database, but it does
give you an unlimited day's travel each day for £3 (or, if you can
change the card's serial number, not even that).