The railroad allowed for easier westward expansion, so more settlers were coming and a lot faster. It was cheaper to freight supplies by train, and they could carry more, decreasing the cost of goods, which in turn increased population with the booming economy. The increase in population led to more towns being established and when connected to the railroad their populations began to soar also. As more and more land was taken up to support this growing population, the Indians lost their hunting and farming grounds, and were forced into desolate areas that weren't fertile and had little game. Without food, the Indians had to either integrate into American society or starve to death.
I hope this helps! This is just my take on it. There are many other contributions the railroad made to end the the Indian way of life, but this is just the first that came to mind.
