I also have issues with cargo in all my games. It especially happens if you try to deliver from one source to two different industries and the travel time differs.
For instance if i have a forest delivering to a samwill close enough by trucks, that route will start developing and reach max potential really fast. However, if i try to deliver fom the same forest to another samwill using train over a longer range, everything goes to hell. The business MAY recover in 50 years or so, but the second sanwill never reaches full potential.
Another issue that I found repeatedly is when trying to supply the same city from two different sources (refinery and samwill). That almost never works, especially if the supply route is two long range trains.
