John Levine wrote:

But to return to the original point of this exercise, to get free
train travel, buy a $20 Visa gift card for cash at the supermarket,
and use it on the train. (Do they even have gift cards in the UK? If
so, make it a 20 quid gift card.)

Until you've bought $20 worth of tickets, it works normally, and the
ticket price is deducted from your balance when the transaction
clears. After that, the bank rejects the transaction, but if the
guard's ticket machine doesn't validate in real time, by the time that
happens you're long gone, and since the card is a bearer instrument,
they have no way to know who to go after. Repeat indefinitely until
the expiration date on the card.

Knowing the BIN ranges of debit cards and gift cards doesn't help
here, since many of them are entirely valid and the train company
will get paid.

Stephen raised the spectre of gift cards to dispute the point I made
that card types were known by number ranges, so you just need the list
of known closed accounts when using a hand-held point of sale device
carried by the conductor, given that authorization won't be convenient
or possible when using these devices. Even you agree that yes, these
are issued in known number ranges. If the transaction with a gift card
cannot be authorized in certain circumstances, then don't accept it
in those circumstances. Similarly, they're not going to accept cards
issued by merchants to give credit to their own customers for purchases
at their own store, like gasoline credit cards. The accounts are issued
in different ranges.

This Web page discusses payment methods that also apply to paying
on train: http://www.nationalrail.co.uk/times_...t_methods.html

Credit/Debit/Charge Cards

All National Rail train companies accept the major cards
such as Visa, Visa Delta, MasterCard, Maestro and Amex.
Some train companies also accept Diners Club International,
Solo and Electron.

Nothing on this page says that gift cards are accepted. Gosh. They
must know the card number ranges!

So many followups later, Stephen's point was a non-issue, but I won't
likely live long enough till he withdraws it.