That's a great photograph, at least in the sense of capturing action that is well-known, but not usually photographed. Not so fun for the ferroequinologists among us.
In the center of the photo there's a large, fixed platform -- roughly the size of a flat car, but this one's not on trucks; it's raised on posts. The stuff on top looks like a rough pile of odds and ends, none of which is easily identifiable. One possible exception to that is what may be the frame of an old hand car, or maybe a very old-fashioned railroad truck, sans wheels and upsite-down under the big pipe. The big cylinder on the right looks to be a pressure vessel (i.e., boiler) of some sort, but the lack of any visible openings on the near end seems odd.
Much more important is the activity in the left and foreground of the picture, where the soldiers have stacked rails over fires. These are being heated in the center and, when sufficiently hot and malleable, bent around the nearest tree or pole:
The big cylinder on the right looks to be a pressure vessel (i.e., boiler) of some sort, but the lack of any visible openings on the near end seems odd.
That sort of arrangement wouldn't be unusual today with U-tubes, multi-pass heads and the like. I'm not sure how many varieties of boiler layout they had available in the years leading up to the ACW.
There could be an opening on this end of the shell side, but not facing up, probably directly away from the camera as there is a seam apparent on this side in the shadow.
The other long tube appears to be a tall flue for something, with bent tabs still visible.