Lots of good advice, but I don’t think it’s true that “people travel from one zone to another. There is no specific destination.” Select a sim person: they each have a specific building in a specific town for work, home, shopping and leisure, and when they take a trip they emerge from one building and travel to another. What I haven’t tried yet is to follow an individual to see whether they persist in the sim after the end of a trip: do they later emerge from the same building they last arrived at and go to one of the others on their list?
I have observed the lack of local traffic to industrial areas that many above have mentioned. I don’t understand it but I do note that residential buildings have an unemployment factor. Presumably the easier it is to get from home to a workplace, even in another town, the fewer unemployed there will be. Whether this becomes enough to make a line sustainable is another matter – I certainly struggle but I think my frequencies are too low and my stops too close together.