I think I don't yet really understand the concept.
In my games I frequently see that the passenger success rate as stated in the buildings screen is highest in towns that are not connected to my network. Sounds odd, but that is what the numbers say. Here's a screenshot: http://i.imgur.com/BzRTIVr.jpg.
The town is not connected - yet has higher passenger succes rates (both local and non-local) than most buildings in my connected towns.
It is not immediately clear why the passenger success rates in your transport served towns are lower - it might be to do with anomalies resulting from the distance ranges for passengers, which distance ranges are due to be abolished in version 12.0.
However, the passenger success rates (which will be modified in the next version) are a percentage of all those passenger trips generated by that building that ended in success - i.e., where the passengers were able to get to their destinations, whether by taking public transport, walking, or taking their private car (if they have one).