I went to college in SD many moons ago. Trust me, there are very few rail lines in SD that could support passenger trains without a total rebuild. The two major exceptions are the BNSF in the far southwestern corner of the state which was the last passenger route in that state and only passes through one small town. The other is the BNSF route from the Twin Cities through Wilmar, Marshall, Sioux Falls, Sioux City, Omaha, St. Joseph and Kansas City. I rode this line several years ago and BNSF maintains it to at least class III standards. It was a very smooth ride. The exception is Sioux Falls to Sioux City which would need serious work. BNSF line deadends in Sioux Falls from the north and a short line controls the line between those two cities. Sioux Falls is also the largest city in the state. The DME has put a lot of money into it's line in the past few years, but the ground west of the Missouri River is unstable. They have upped the speed from 10 to 25, but would require big bucks to get any faster than that.