Hi Great-Northern-Fan54. I do not have TS12, so can not speak to it, but I can tell you there are not a lot of PNW routes in Trainz that I have found.
Tume's Avery-Drexel covers the Idaho Panhandle and part of Western Montana, but that is for T:ANE or higher, I believe.
The free TS2009 has a Maria's Pass Approach route in Western Montana, that should work in TS12, but you may have to get the 2009 version to get the route.
There is also the Port Tillamook for the newer versions, but not for TS12, I don't believe.
There is also one called "0 Great Northern Railroad Scenic Division 1893-1900" by Funnnyfarm that is a TRS2019 route he did not complete between Wenatchee, WA and Everett. The east side is mostly done other than the roads and rivers and it looks great. It has switchbacks at the summit, then trestles, snowsheds and tunnels going down the west side. But development ends before you get very far down the west side, although I think track is laid all the way to Everett.
It would be great if there were more in the Northwest, it is beautiful country. Tume has hopes to develop the Milwaukee road line from Othello to Tacoma, but he has to finish his work in Montana first. Again, it will be for the newer versions.
If you want help from others, contact the Help Desk above and see if you can get your copy of TS12 registered on your timeline.