There are several things to look at in this photo. These are Burros, not donkeys or mules. an average height would be 3.5 ft to 4ft max at the shallow of he backs. The timbers, ties or mine supports are 5.5 ft to 6ft in length. There quite thick for the length to be a more modern cut tie. It was faster and cheaper to leave them the thickness of the tree than to mill them down. As per the age or time period look at the size of the rail? Now for the where? I all ways get these wrong, but this photo feels like the east side of Marshall Pass. Also for size comparison take a look at the two fellows in the back ground who appear to be standing lower than the burros in front of them.