In article , wrote:
I wonder if an alert goes off if a denied card has been used recently and
the person could still be on the bus. Or whether for 1.50 its not worth the
hassle of calling the police.

What are they going to do -- stop the bus and frisk everyone until
they find the wicked card? At whicn point the holder will (quite
possibly truthfully) say, oh, sorry, I meant to use my other card.

There's a reason that transit systems increasingly use proof of
payment rather than gates.