While the Industrial Revolution started in the late eighteenth century with the development of steam technology, industrial development only became more widespread in the second half of the nineteenth century. The railroad was certainly an important factor in the growth of industry, but there were others.

One factor was what we can call the Second Industrial Revolution. While the first was centered in Britain, the second was driven by US innovation. It was marked by developments in the chemical and electrical industries. Think here of Standard Oil and of Thomas Edison. It was also driven by mechanization and standardization. Think here of Henry Ford and the assembly line.

Politically, this was the age of the robber baron and industrial growth was partly driven by a highly competitive marketplace still largely unaffected by government control or antitrust laws. This openness arguably allowed for innovation and growth in industry.

These factors, and others (e.g., immigration and the influx of a cheap labor force), helped spur on American industry to dramatic developments in the late nineteenth century.

American industries grew dramatically partly as a result of the first two factors you mention and partly due to other factors not mentioned here.

During this time period, many new methods of producing goods and of organizing and running businesses were coming into existence.  These methods helped industry to grow.  One way they did this was by encouraging the growth of railroads.  The railroads helped industrialize the country both by creating demand for things like steel and by linking various parts of the country to create a bigger market.

But these were not the only factors.  There also political factors.  The government was very pro-business and put few limitations on its growth.  Just as importantly, there was a huge boom in immigration during this time.  These immigrants provided the cheap labor that was needed to drive industrialization.