The BNSF Point Defiance line has not had any major mudslides that closed the line in quite a few years now. As the line is double-track except for the tunnels under Point Defiance & Ruston, most such slides would only affect the track closest to the hill side. The BNSF line from Seattle North to Everett has several single-track segments, ironically right where the most active slide areas are located. This line has had several closures and passenger train restrictions each of the past 4 or 5 winters.
If indeed Amtrak intends to keep the Coast Starlight on the current route ( Pt. Defiance line ), then they would have to keep the current Tacoma Amtrak station open as well as having another station @ Freighthouse Square. This would be a ridiculous duplication of services and a waste of money besides.