what is the threat the iron horse presents to the Kiowa?

Little Wound and Pawnee Killer compared the railroads in the sixth chapter with an iron horse, highlighting the idea that they were separated from the modern world. For them, the railroads had no use and they could not understand why the Americans would go to such lengths to kick them out of their lands. By comparing the railroads to something that was familiar to them, the Indians tried to make sense of the world foreign to them.