The criteria is the 20 minute radius, simple by that.
For example the steel mill, coal mine and iron mine.
If it takes longer than 20 minutes for the coal and the iron mine to deliver their ores to the steel mill by themselves, they don’t deliver it. So it can happen, that an iron mine produces nothing even in the 2000’s.
You can start it up though, because I have this case in my game.
I created lines between a steel mill and a coal mine and an iron mine. All 3 were producing nothing, the steel mill because of the lack of ressources and the mines because the way to the steel mill took for their goods longer than 20 minutes by themselves.
The train lines, though also a road vehicle line should do the trick too, created a fast way and thus all 3 started producing their product, the mines their ores and when the ores reached the steel mill, it produced goods.