I may be going from central London to Amersham at the weekend - if so
will already have a travelcard which covers zones 1-6. I realise
Amersham is in Zone 9 and don't want to be clobbered with a penalty fare
on arrival. I would be happy to pay the excess from zones 6 to 9 by
Oyster Card, but I don't see how to do it. The train is hardly likely
to stop at a station in the boundary area long enough to allow me to hop
off, find an Oyster Validator, and hop on again, but with only about one
train every 30 mins, waiting for the next one isn't attractive.

I can't find anything about this on the TfL website, but it must be a
non-uncommon problem. Does anyone know how to solve it?

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Clive Page