In message , at 15:35:47 on Thu, 7 Aug
2014, Michael R N Dolbear remarked:
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).

Stops dead because they moved Z6 (NR only) to suit.

So Hampton Court, Tattenham Corner and others

Why not also Brentwood, or did they want to differentiate it from
Shenfield.
--
Roland Perry