In message

..net, at 20:57:49 on Tue, 18 Dec 2012, Neil Williams
remarked:
"Finished your journey? Touch the purple validator to ensure you
are charged the correct fare".

Or perhaps more logical one to signify that you are continuing it.

Even better, just close the journey if a touch-back-in doesn't occur within
the OSI time period, if necessary the next time the card is used. Don't
see why that's hard.

I think you misunderstand the failure mode, which I think is people
leaving the OSI station to go about their business, and who then return
for an onward journey *before* the OSI times out, which then confuses
the system if their A-B-C (where B is the OSI) exceeds the quota for an
A-C journey.
--
Roland Perry