The construction of Railroads are is considered to be America's first big business that helped with the development of other industries
Answer: Construction of Railroads
Explanation
The introduction of railroads changed the face of the American economy.
It flourished the trade business along with its development.
Transportation of goods across states where railroads present took place pretty soon after the introduction, thereby increasing the stability of the economy.
Production and transportation of goods like raw iron, steel and coal became influential in boosting the respective industries.