A line of small square rooms of some sort. Maybe 10’ to 12’ to a side. Half buried, and just a door in each. I would say the ruins of old dungeons, but that’s probably not it 
I’m going to say a row of greenhouses, where all each has missing are their roofs, but the peaks high enough for the doors to reach full height. Kind of like this only smaller and next to each other.
Hmm? It’s in Nebraska, yes? My son has two theories: a) a cellar for provisions.
b) some type of primitive tornado shelter. Assuming of course that it had a roof at some point.
I’m not too sure on this second guess. Are you under the impression that it never had a roof? If they didn’t, then it was not used to store anything either.
Is there any reason they might not just be pig sheds?
Sounds like the right size for horse stables?
Something like that. The fact that they are half buried in a bank may be nothing more than convenience of location (or to hide them from immediate view).
What’s the big rusty cylinder nearby? Feed silo?
It looks like a foundation of an outbuilding to me. Imagine a second, wooden story atop the concrete. You can see fasteners atop the near wall sticking up every few feet, where you’d attach a sill plate.
The interior concrete walls were probably load-bearing, and then you could store some fairly heavy stuff on the upper floor - easily accessible from the top of the bank. I’m not enough of a farmer to know what exactly it would be used for, but maybe pigs on the bottom and feed on the top.
I see similar constructions in the West for containment of gravel, sand, sawdust, etc.
The railroad once went between the rooms and that cylinder, it might be an artifact of that.
I was thinking along with Miss_Gnomer and a horse stable. A couple extra feet of timber would make enough headroom for windows for air circulation when needed.
Phu Cat
My guess: first floor of a 2 story building. The rooms on the first floor look like they were used for storage of some sort.
The solid walls look like the kind of walls you would use for a foundation (load bearing). The hill on the back seems to indicate why the structure was built next to the hill: easy access to the 2nd floor.
So the first floor was both storage rooms (like a basement), with the floor of the 2nd story as the ceiling.
You entered the 2nd floor from the hill side, but accessed the first floor from the opposite side (via the doorways).