I think the logic is that they only show up at the station if there is a train that goes where they want. Much like in real life how the trains go on a schedule

I think the logic is that they only show up at the station if there is a train that goes where they want. Much like in real life how the trains go on a schedule and you get on if it is going where you want. Now I'm sure the simulation isn't perfect but it seems to make enough sense for game logic.