In message , at 01:07:41 on Fri, 5 Feb
2016, John Levine remarked:
So how does the system deal with someone entering at an ungated
station to exit at another ungated station,
carrying a pre-paid card with a zero balance? If gripped it seems the
RPI won't be able to tell whether or
not they touched in and whilst the overnight reconciliation would try
to charge a maximum fare for an
unresolved journey but this would be declined due to the zero balance.

It probably doesn't. Keep in mind that the goal isn't necessarily to
have £0.00 in lost revenue, it's to get as much net revenue as
possible. The savings in not having to sell and manage zillions of
Oyster cards likely would pay for a fair amount of contactless
shenanigans.

In the worst case, they could just stop accepting prepaid cards other
than Oyster. The prefix of the card number identifies the type of
card.

That'd be a bad move for tourists as many of the cards which are issued
in foreign (to the tourist) currency are prepay Mastercards. Although
they are not ideal for a party travelling together as you require one
each.
--
Roland Perry