Ian Clifton wrote:
"Richard J." writes:
A "sensible & perfectly informed passenger" would know that you can
get from Gare de Lyon to Gare du Nord by RER D without the need to
change at Châtelet-Les-Halles.

Apologies, I think I misremembered a change.
(Also, I have a feeling there are through trains from Bourg to northern
FR/BE/NL in the winter, which might skew things somewhat)

I think I intended something like Theo’s solution to be valid. But
you’re right, in that calling my Passenger “sensible” is inadequate and
probably misleading. A sensible person would probably make a longer or
more expensive journey, rather than endure the faff of many
changes. Maybe it’s better to make him an automaton: the Passenger
always boards the very next train that gets him at all closer to the
destination (even if a following train would get him closer still, or
would reduce the subsequent changes required).

That would make it quite sensitive to time of day and service pattern, which
would make the challenge rather fragile.

Freedom to pick any train on a given day might make sense - the traveller
can start at 00.01 and take any combination of trains that gets them there
by closedown the next morning. But they can't wait for the one through
train a week. Or you could look at trains with a regular service pattern -
perhaps the last train is an all-stations stopper, but during the daytime
you'd have to change.

The next question is whether the passenger must travel on the same line as
the through train, or whether they can take an alternative route (in my
example, you'd probably have to use the around-Paris HSL rather than going
into Gare de Lyon). I'd suggest the station should be one passed through by
the through train, but they passenger need not take the same route to get
there. (So for Inverness to Finsbury Park the traveller could
change at Stevenage and take the Hertford Loop).

Theo