This would be C. 1830- Baltimore and Ohio Railroad :)
During this period, the industrial growth of the United States had great effects on them. Cities have become the country's main economic centers, and the manufacturing and finance industry have overtaken agriculture and livestock as the main sources of income in the United States. The industrialization process drastically increased rural migration. At the end of the American Civil War, about a quarter of the American population lived in cities. In 1918, half of the country's population lived in urban areas. In addition, this period is also marked by the large and unfounded immigration of Europeans to the country.
Http://www.theday.com/article/20120628/NWS01/307019996
Recruitment by the railroad companies
