On page 10 of the rulebook there is a diagram to illustrate using other players' track. My understanding of this is that you may not use other players' track entirely to connect two cities on a passenger or national card, and other cities between those on the route card are irrelevant. Put another way, there must be at least one of your track segments between any pair of cities on the route card. Is this correct?
From the rules: "You cannot use other players’ track
if the route does not include any of
your track whatsoever."
It doesn't say anything about individual cities. The example is pretending that Dresden to Hamburg is a route.