Loose the land for other things and then starts more land and room for slavery
The transcontinental railroad transformed the west because it made the idea of manifest destiny so much easier to achieve. The railroads also gave access to many trade spots, which in result connected them all, making the economy of America BOOM.
With economic stability came the need for warriors, as food became more plentiful, people were freed up to serve as community leaders, and as settlements developed, leaders emerged to help organize and regulate these budding civilizations. Hope this helps.
