We had a Reddit topic about this earlier today/last night, that was sorta explaining things. Basically, things need to be close and of course within a 20min distance, or they just gonna avoid your line.
http://www.reddit.com/r/trainfever/comments/2fhz1f/how_the_hell_does_cargo_work/
Didn’t have a great example in my current game, so here I made a quick [proof of concept](http://i.imgur.com/J2fzaJ5.jpg) for you. The wood is delivered from the Forestry on the left to the sawmill on the right through the chain of Trucks -> Train -> Trucks.
Note that the goods produced in the sawmill also automatically make use of the train line to make a small shortcut on their route to a town in the south.
Is how one mate explained it to me effectively.