Colin Rosenstiel wrote:

Banstead and Epsom Downs
Chipstead, Kingswood, Tadworth and Tattenham Corner

What's the logic in extending Z6 to Epsom Downs and Tattenham
Corner, but not to Epsom? Are there particular difficulties caused
by Southern 'sharing the flow' from Epsom with SWT?

Not sure but there's also a different district council involved which
*might* be a factor.

Hardly so. The Public Transport authority will be Surrey County Council.

True formally, though the fears some in Epsom & Ewell, including but not
just borough councillors, hold about anything that looks like the next step
towards incorporating Epsom into Greater London should not be
underestimated.

One other point is that never to my recollection have Epsom and Epsom Downs
been treated as "Epsom stations" with tickets valid for either and they
really can't be used as an interchange in any meaningful sense (unlike, say,
Canterburys East & West). Epsom Downs serves a combination of semi-rural
suburbs and a school and has always had separate fares from Epsom.

Also just Southern extending Zone 6 to Epsom would create the problem that
one Ewell station would be within the zone and the other wouldn't - and I
think there has been a "Ewell stations" use in fares. Plus it's debatable as
to whether Epsom alone on the South West Trains network should be
incorporated - what about some parts of the Cobham line or Shepperton? - and
equally should the zone stop at Epsom or carry on to Ashtead (still in the
M25), Leatherhead (occasional terminus) or even Dorking?! This is of a
different order to two backwater branches.