multi good and multi stop cargo ships?

So right now I have 3 locations all right next to a river so i figured i would setup a construction materials line. My line starts where a truck bring down rocks to the harbor. the boat picks up those rocks and takes them to a harbor near the construction factory where a truck brings them up the road to the factory. this part of my line already woks fine.
I want to then have the same boat pickup construction materials and goto the last city on the line and offload them. right now i cant get the construction factory to say yes to line use. All my harbors and truck stops are basic since im playing in 1850 and quite poor. the end goal city where i want the construction materials delivered has a harbor that is currently receiving food and that food is being trucked to a central truck station which is also basic.
what do i need to do to get this going? do i need a large truck stop at the factory so that it can pickup rocks and bring back materials from the same stop?
do i also need to upgrade the main truck dump which is currently only receiving food from the harbor?
this game is fun but they really dont offer a single bit of help in solving their system problems. it seems pretty basic to me but i cant get the line to work.