The western American frontier
("Wild West" is a bit of a modern imagination) was largely the settling of territory west of the Mississippi River after 1850. The Pacific Railroad Act of 1862
marks the point where the government encouraged private capitalists to start building out railroad infrastructure on the West Coast to reach out towards the Central Plains and create a transcontinental railroad link across the nation. The link was completed in 1869
, afterwards rapid urbanization and industrialization took place, first in the small urban areas that served as crossroads for the railroads and the various land route providers such as Western Union, Wells Fargo and Pony Express, then the West Coast proper as seaports were developed to engage in trade up and down the entire Pacific Ocean. Within 20 years urbanization had largely brought the western united States up to the same standards and class as the eastern United States. In 1890 the superintended for the US Census stated that there was no longer a "western frontier" aside from a few spots in the middle of the US that were still territories and were not as well organized as the rest of the nation.
Globally, 1870 represent the general start of the Second Industrial Revolution
, technology and industry found synergy in advancing railroads, mining and manufacturing, and the start of industrial advancement for communication, electricity, petroleum, chemicals and the applied sciences.
I'd suggest that if you want to represent a more rough, challenging start to your scenario that you start it in 1850 or 1860. This will give you quite a bit of time to build infrastructure before the roaring '90s come along!