Mizter T ) gurgled happily, sounding much like they were
saying :

~ Would Chiltern operate their own zonal system, a bit like zones A-D
on the Metropolitan line?

And there you touch on one possible reason they've done this... that they
already accept Oyster PAYG on the gates at Marylebone, because PAYG is
valid on the Aylesbury via Amersham route as far as Amersham.