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.
This is an important thing I’ve noticed. The industry’s production is determined by the demand. They won’t just keep producing if the cargo isn’t going anywhere. So like in the case Azrael mentioned if the iron mine doesn’t have a buyer it won’t just produce and stock pile it, it will just sit there producing nothing at all until it can send it’s materials to a buyer (steel mill) within 20 minutes of travel time (be that walking, bus, rail or a combination of all three). It is really a ‘pull’ system, not a ‘push’ system like you may expect.