The thing I would consider too is that the dramatic decrease in buffalo numbers forced the plains tribes into smaller and smaller clusters. It was this effect that made the Plains Wars of the late 1860s and 1870s strategically viable with reasonably small numbers of US troops. Although Gold found in the Black Hills was the rationale for the campaign it is pretty clear the extermination of the plains tribes (either via forcing them onto reservations or through conflict) was the long term aim in US Government Policy. So the impact of the railroad on the Indian Wars and the subsequent land left for settlement because of this is something I would consider. I don't get to teach this lesson at the moment but would love to. Good Luck.