How did the completion of the transcontinental railroad chane the lives of american citizens?

It substantially accelerated the populating of the West by white homesteaders, led to rapid cultivation of new farm lands. 
The railroad industry also wanted bison herds culled or eliminated. Herds of bison on tracks could damage locomotives when the trains failed to stop in time. Herds often took shelter in the artificial cuts formed by the grade of the track winding though hills and mountains in harsh winter conditions. As a result, bison herds could delay a train for days. 
In the east, the progress started in Omaha, Nebraska, by the Union Pacific Railroad proceeded very quickly because of the open terrain of the Great Plains. However, they soon became subject to slowdowns as they entered Indian-held lands. The Native Americans living there saw the addition of the railroad as a violation of their treaties with the United States. War parties began to raid the moving labor camps that followed the progress of the line. Union Pacific responded by increasing security and by hiring marksmen to kill American Bison-which were both a physical threat to trains and the primary food source for many of the Plains Indians. The Native Americans then began killing laborers when they realized that the so-called "Iron Horse" threatened their existence. Security measures were further strengthened and progress on the railroad continued. 
European Americans would often shoot buffalo for sport from the train; by 1880, the buffalo were mostly gone and Plains Indians had been gathered onto reservations. Millions of acres of open grassland were being settled by the people moving west. Eventually, much of this land became the farmland that fed a growing nation.