As americans moved west, they built the railroad and then many new towns and cities. also, as americans moved west, the buffalo population declined. why?

This question is quite tricky, either you are talking about the city of Buffalo or the Buffalo animal. Either way, the population of these species started to decline when they moved West because these animals were mainly used for the fur trade during that time period. Buffalo City is known as such because of the trade practices that people do during that era.