On Tue, 26 Sep 2006 17:52:52 GMT, Richard J. wrote:

Actually the PAYG fare would be for Zones 3-6, otherwise he would
not have a valid ticket between Northfields and Boston Manor
(across the zone 3/4 boundary). The costs are the same, though.

Actually it would be for 4-6, because his travelcard covers zones
1-3, and so the extension would be for the additional zones he is
travelling through - zones 4-6. Otherwise he'd be paying twice for
zone 3.

The travelcard covers his journey as far as Northfields (zone 3) and not
beyond. He then needs an extension which is valid from Northfields to
Heathrow, which has to be a zone 3-6 single. Your logic would work if
Northfields were a boundary station in both zones 3 and 4, but it isn't.

As others have said, this is not how it works in practice. If you
like, the Travelcard is valid to the Z3/4 boundary (a point on the
track somewhere between Northfields and Boston Manor), and only a Z4-6
single is needed to cover from there to Heathrow.