I notice that sometimes my passenger trains going over a route that initally starts out at 0% occupy goes up to 100% or more in an hour or less, and sometimes can go on for 8 hours or more without increasing route usage dramatically. What determines the demand for passengers for a given route? Some factors that I can think of that may or may not have an effect are:
1. distance between stations (is there more demand for nearby stations than faraway stations?)
2. City level
3. How quickly trains do a route: for example, since I got Minerva in my current game, routes get full usage very rapidly. But I put my bonus trains on a separate route and they can often go a lot longer --- in this case, there may be advantages to using slower trains (like Talos) vs. fast trains (like Minerva).