Enough details can be observed to give one an educated guess that this is the original bridge over Douglas Creek, between Columbia River and Vulcan.
This line was replaced in 1902 by the current line, still in use by BNSF.
See this location today.
Mac McCulloch suggested the following:
"I agree that a switchback as drawn is consistent with the photo.
"Why do that? Need construction materials to base of the bridge. Perhaps but seems like a lot of effort compared to a wagon road.
"Expect a long delay to build the bridge and want to leap frog materials to a point just above Rock Island dam or to Wenatchee. Getting material to Wenatchee makes sense. Needed a steamboat landing at South Wenatchee anyway and if got supplies there GN could start West without having to wait for this small bridge to be built at Douglas Creek and for the time to build 15 miles or so of line along the east bank to opposite South Wenatchee. Hill was in a hurry, but that big a hurry??
"Either option required passing Rock Island with steamboats, a high risk operation. For enough money I suspect some captains would have tried it, particularly if traffic to the Okanogan mines was slow.
"Supplies to Wenatchee would be my guess. Perhaps Ellensburg papers of the day have an article that would explain it."
Ted Curphey figured the spur below the bridge in the center, and then in the creek behind the bridge in the left of the image was a switchback. He put together a map: