CiMfolk decide to make a journey, then they pick a method of transport to get there. They can walk, take a taxi, drive their own car, or use your Lines. Once they plan their journey, they don't ever change their plans.
If you build a Line (let's call it Line 1), which goes from A to B to C to D and back again, people who are travelling between A,B,C and D can decide to use Line 1 for their journey. Suppose Line 1 gets very busy and there's 150 people waiting at A, and 100 of them are going to D.
If you build an express Line 2 which goes between A and D only, none of the passenger waiting for Line 1 will notice. They decided, before they began their journeys, to use Line 1 and nothing can make them change their minds.
But the NEW passengers, who start their journeys after you've built Line 2, can decide to use Line 2 to get from A to D. I'm not sure if they will guess which line is quicker, or if half will choose Line 1 and half will choose Line 2.
If you really want to get rid of that big crowd of people stubbornly waiting for Line 1 to take them from A to D, there is a solution. Remove stop A from Line 1, then put it back in again and restart Line 1. Everyone waiting at A will magically teleport away, and then begin new journeys where they can choose to use Line 2 if it suits them.