Friends of mine in the employ of Amtrak have mentioned that Amtrak is not all that interested in using the Prairie / Ft. Lewis line. It is Sound Transit that is pushing for Amtrak to reroute so Amtrak can help pay for the improvements made to the line. As noted above, BNSF does give preferential dispatching to Amtrak, at the expense and delay to BNSF & EwePee freight trains on the current Pt. Defiance line.
I also agree with those who see impending problems with instituting a medium - speed, multipule passenger train operation on a currently sleepy, stub - ended branch that sees one 10 MPH local freight per week. Shades of the Baldwin Park branch !!! Darwin will be busy...