There's some evidence that Brentwood and Broxbourne form a virtual "Zone

10" (see page 15 of your guide) and possibly Theobalds Grove and Waltham

Cross a "Zone 11", with Ockendon, Chafford, Purfleet and Grays in a

"Zone 12"...



Any expansion plans are (conceptually if not for implementation) as

simple as there being many more sets of stations like Harold Wood and

Brentwood identifiable so they can work out the right fare to charge

from Shenfield variously to Harold Wood, and new places like Luton

Airport and Dartford and perhaps other similar ones just outside the

current limit like Swanley, Esher and Hinchley Wood.



If we had a uniform pricing structure across all TOCs then it would be

easier I'm sure. Unfortunately we don't outside the zones so while I

agree your concept is fine it doesn't align with reality.



If we add Shenfield, Chesham and Broxbourne as one-station "virtual

zones" that brings the total to the 'limit of fifteen' mentioned last

year. [And incidentally scupper the four extra stations out to Hertford

East].



My theory then, which I'm beginning to think is too simplistic, would

require some kind of new coding to cope with any additional stations,

and it's odd that National Rail acceptance on all the lines other than

the ones above stops dead at exactly the edge of Z6, when some of the

currently proposed extensions would make a lot of sense (eg extending

one stop to Epsom).



One fly in the ointment though is the Overground to Watford Junction,

where there have been "Special fares" to Watford Junction (a cludge to

get round virtual Zone 16?) but now that Oyster is accepted at

Carpenders Park, Bushy and Watford High Street then there has to be some

scheme already in place for "19 zones".



The next phase of extensions (proposed and actual) will require being

able to recognise and do the sums for these stations outside Z6:



Dartford

Epsom

Cuffley, Bayford, Hertford North

Radlett, St Albans, Harpenden, Luton Airport Parkway (&Luton?)

Merstham, Redhill, Earleswood, Salfords, Horley, Gatwick

Potters Bar, Brookmans Park, Welham Green, Hatfield, Welwyn Garden City





Theobalds Grove and Waltham Cross are in zone 7.
Cheshunt is zone 8.

http://www.tfl.gov.uk/cdn/static/cms...rvices-map.pdf


Potters Bar would have been in zone 9 under the cancelled FCC Oyster extension plans.

Poorly programmed ticket machines have revealed that Watford Junction is "Zone 10", Broxbourne and Brentwood are "Zone 11", and Shenfield is "Zone 12".