ian batten wrote

What happens if there are two station, one of which has a machine
which is designed to take only cards

Then it is in full working order if it only accepts cards, although
it's not clear that such machines exist.

They do, although I've only seen them for definite at some big terminals.
However I must admit I don't tend to look too much at ticket machines these
days.

However, if you boarded a
train without a ticket, made no effort to find the conductor, and got
gripped some time later, then fairly obvious inferences can be drawn
about your intent to pay.

A conductor is a very rare thing on trains in London, to the point that one
can reasonably expect a train to *not* have one on them. On the rare
occasions I've had to use PTTs (my local station has the ticket office in a
side room from the entrance hall, when closed the room is sealed off and
there was a period up until a few years ago when all the TVMs where in
there) I've never expected to be able to sort things out on the train.
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