I've been trying something a bit more ambitious as an industry chain
- Oil Field to Refinery
- Refinery to Printing Works (chemical)
- Plantation to Paper Mill (lumber)
- Paper Mill to Printing Works (paper)
- Printing Works to Publishing (books)
And the less i can say is that it's unreliable, there are months i manage to get +500 of profits and there are months i get -1500 of profits, the reason being when an industry reach full stock, trucks stop to carry and sell stuff and get "Stuck" (i thought i was a bug until i noticed in depending industry they had full stock already, trucks will re-work only when there are more space made) , but i still pay the maintenance and operations.
Hard to balance things to get only profits without the nasty "we're full" months that break them.
The problem is that the bad "we're full" months can last for 2 or 3 months, meanwhile operation and maintenance cost a lot (especially as i had to make roads to build the required network).
EDIT :
Selling everything out of the original line (oil field to refinery) it looks like the real maintenance killer are the roads.
As after selling all the roads that weren't needed for the original line, profits were back.
My problem there was that i really made the minimum amount of roads as all those industry weren't close of the pre-built ones.
I can't see how to work around this , as i guess trains would be even more expensive than trucks there.
edit 2 :
And now the refinery is full, does not consume anymore its own oil and so does not require any .
Hmm, looks like for more constant profit you'll need to switch to other routes regularly and sell the previous ones, and rebuild them once they're having space (and sell the ones you had switched too).