In fact, the greatest impact of post war railroad construction was the development of the West as well as the creation of a truly national economy. Congress took the initiative while southern states had no representatives in Congress to authorize a truly trans-continental railroad. Once completed, the railroad cut the time and cost to ship materials cross country to a fraction of the previous amount. Railroads carried the mail either for free or at a reduced cost, which also helped stimulate the economy. Railroads received huge grants of land along their rights of way to construct watering stations, depots, etc. These stations soon blossomed into towns with restaurants, hotels, etc. Cattle drives terminated at railroad stations along the west. The end result was the development of towns and communities in the west which would have not taken place as rapidly were it not for the railroads.
Railroads were the first true "big business' in the U.S., and the first business tycoons were railroad owners. The impact on economic growth of the U.S. is impossible to overestimate.
The major impact of railroads was to make transportation of goods and raw materials faster, easier, and cheaper.
Before the railroads, it was very difficult to move goods by land. The only cheap and easy means of transportation was by water. This meant that trade had to follow rivers, lakes, and canals. The coming of the railroad changed this. With railroads, goods could be moved across land cheaply. The link below tells us that transportation prices dropped as much as 95% between 1815 and 1860, largely because of railroads. This helped the US economy develop because it broke down barriers to trading between places that were not connected by water.