Railroads have a lot of challenges when crossing new terrain, but unplowed prairie is not one of them. They had no need to "soften the earth." In the 19th century, railways could and did simply drop their wooden ties on the ground, and perhaps later add rock ballast on the roadbed to cushion the rails as they flexed under heavy locomotives, and secondarily, to control weeds within the roadbed. Any grading that was done was for elevation (to avoid steep slopes) or drainage.

Rail lines did often follow the traces and wagon roads that early pioneers and settlers had used—but that's merely a case of them following the logical route through a mountain pass, along a ridgeline, or next to a river. The same path might have been found through the landscape first by animals, then used by Indians, then widened and confirmed by settlers and wagon users, and finally built upon by railroads.