The railroads were integral to the growth of the United States during the latter half of the nineteenth century in several ways. First of all, they allowed people to more easily, safely, and affordably travel to and settle the western part of the country. Before the completion of the transcontinental railroad in 1869, there were only two rather unattractive ways to cross the country. People either crossed overland by stagecoach or wagon train, which was long and dangerous, or they went by sea with a transfer in Central America, which was very expensive. The expansion of the railroads changed that. It almost immediately brought in large numbers of settlers from the east. Soon, there was a large enough population in certain areas to create new states, such as Colorado, North and South Dakota, Wyoming, and Montana.
The railroad helped economically as well. It meant that resources from distant places could be moved to production centers in cities. Iron ore from the Rocky Mountains was transported east to steel mills in cities like Pittsburg and Chicago. Crops from California could reach markets in the Midwest and East before they had time to spoil. As raw materials and consumer goods moved quickly and inexpensively around the country, the growth in consumerism grew greatly, fueling the growth of the American economy.
In short, the railroads facilitated the fast and cheap movement of people, products, and ideas around a large and growing country. Without the railroads, obstacles such as mountains and deserts isolated large parts of the country from development and growth. With the railroad, the country became more connected than ever before.