I don't know about the siding in Whitney Valley, I have not seen reference to it on the ROW plats. As for White Pine Junction, there was never a siding there. The West bound trains would shove the empties in on the West leg of the wye, and the East bound trains would pick up the loads from the East leg.
This scene, which has been posted before, shows an East bound pick-up. Another photo of the same train shows the additional loads cut in behind the box cars. We have reason to think these photos were taken by Fred Jukes.