The portion from Shelby to Whitefish is known as the Hi-Line Subdivision. If you want to get that line to Tacoma, you're going to have to build it yourself. There are no addons and I doubt there will be.
You can get the whole route in three sections for free on the DLS.
Then you can merge all of them together and then do all the scenery and texturing and plant all the trees. This takes a while. Or you can get one all done in payware from Josef Pav. My suggestion.
This is probably the biggest route in trainz. And well worth the money. I helped to test this route for Josef and I also textured the first route that he used to show off some of his Rolling stock on.