A national railroad system needs a lot of railroad tracks, right?
And Carnegie seized the opportunity. and he came up with a system of business organization called Vertical Integration. In the case of the steel industry, this means the mining of iron ore, the transportation used to get ore from the mine to the mill, turning the ore into the steel, the manufacturing process and sales. Carnegie controlled all of these, he practiced Vertical Integration on such a large scale that he practically owned the whole steel industry. This, of course, gave him a lot of political clout, just a quick sketch, but you get the idea, right?