The line change from Tumwater to Chumstick Canyon was part of the new Cascade Tunnel relocation and electrification project. Although the route through Chumstick Cyn. required several new tunnels, Great Northern found it was a better route than the twisty line through Tumwater Cyn., where the river tried to reclaim the right of way every spring. Today's U.S. Hwy. 2 was built on the old r of w. BTW, The dam in Tumwater Cyn. was built by GN to supply power to the original electrification project.
Regarding the spur track at the mouth of Chumstick Cyn., this could be the remnant of the original line that GN left in service into " downtown " Leavenworth to serve several fruit packing warehouses. I am not aware of any RR line that ran up Icicle Creek; perhaps there was a logging line ???