I think the misinterpretation is coming from the idea that industries should operate at an imagined “full-capacity” simply because you supply them with enough materials.
As far as I’m aware, Train Fever works rather differently. An industry won’t produce more goods than there is demand for those goods. That demand can be fulfilled entirely by a single industry, which will grow much larger over time to accomodate it, or the demand can be fulfilled by several different industries, in which case they’ll balance the demand between them.
The issue you’re likely coming across is that if you’re completely fulfilling demand from a single industry already, and you link a second industry into the network to cover that same demand, the load will balance between them… you won’t just force them to produce twice the output even if the demand for that output doesn’t exist.
The main reason you’d connect up a second industry isn’t to somehow generate a mass of extra demand, but to help with the logistics of meeting the existing demand. You won’t make any more money directly, but you could stand to save a lot of money through added efficiency over time.
If you want to promote demand, you need to do the things that grow cities… supply passengers and goods… and then wait until they create the demand themselves.
As with anything in a simulated economy like Train Fever uses, you won’t see the results instantly… you’ll likely see losses at first. You have to wait and let the results play out over time.