In message , at 23:17:00 on
Thu, 9 Jun 2016, Richard remarked:

What I wonder is how do they work it out for journeys which one can do
in any of three ways (TfL only, NR only, and both) where there is only a
common gate-line at each end, for example Farringdon to Kentish Town?

In that case it doesn't matter, it is an "interavailable" route and
each operator's tickets are valid on the other (this applies between
West Hampstead and London Bridge or Elephant).

Depends what you re trying to work out. If it's how much you put up the
interavailable fare after NR has had an annual increase (but the TfL
element is frozen) then there's only three choices:

Put it up (breaks Kahn's )
Freeze it and tell NR to suck up the inflation element.
Freeze it, pay NR an inflation rise on its proportion of the fare basket
for that flow, and reduce the TfL income.
--
Roland Perry