The railroads did this in two main ways.
First, a tremendous demand for steel was created when the railroads really started to expand. This was due to the needs of the railroads themselves. There was a great need for steel simply for the rails that were stretching out all over the country. There was a further need for steel to build all of the rolling stock that would travel along these rail networks. All of this demand for steel helped to make the United States much more industrialized.
Second, the railroads created a much larger national market. Before rail, it had been hard to get goods from place to place, particularly by land. With the coming of a huge rail network, this became much easier. Now, raw materials could be moved long distances to factories. Goods produced in factories could easily be distributed around the country. This, too, stimulated industrialization.
Thus, the rise of the railroads stimulated industrialization both directly (by creating a huge demand for steel) and indirectly (by creating a larger market for the goods produced by industries).