The grade between S Wye and Larch from the profile maps has a minimum grade of 2% and Max of 4.08% and stretches everywhere in between. Engines would/did get a workout.
The photo from Mountain West Digital Library of SVRy 17 on the turntable is attributed on my computer to photographer Walter Grande and is in the Pacific NW Chap. NRHS collection. These 3 additional photos are from the same set which were taken Oct. 1939. In the first #17 is backing from S Wye to Sumpter. The second one showing the train ascending Larch looks to be taken from the caboose. Mr Grande must have arranged a ride with the crew. And the 3rd precedes the first two in order of events, was taken at Boulder Gorge with the engine approaching the pile trestle that crossed the river at what was once referred to as Devils Gate.