Final answer:
Railroads significantly contributed to American industrial growth by revolutionizing transportation, boosting related industries, and transforming the nation's economy and society.
Step-by-step explanation:
The construction and expansion of railroads were seminal to the American industrial growth post-Civil War. Railroads became the nation's first 'big business,' revolutionizing transportation and spurring developments in various industries, including iron, wood, coal, and steel. The transcontinental railroad and the associated rail network made it possible to transport goods and people faster and more efficiently than ever before.
By providing a reliable means of transporting raw materials to factories and finished goods to markets, the railroad infrastructure became central to America's transformation from a rural society to an urban one, offering new job opportunities and contributing to mass production. Innovations like air brakes and stronger steel rails improved the volume of traffic that could be handled, allowing railroads to move more freight and passengers across the nation and even foster a national market system.
The societal impact was significant, facilitating the movement of millions of immigrants who came to work in industrial factories. These changes also had a darker aspect, such as the displacement of Native Americans and the harsh realities of factory work, which led to the rise of labor unions.