Boxing a bike on Amtrak is not as difficult at boxing one for UPS or the airlines. The bike boxes sold by Amtrak (cost $10) are quite large. 71 inches by 41 inches by 8.5 inches, though this is probably not long enough for a tandem. You can put up to 50 pounds in the Amtrak bike box (bike only, no other luggage). You have to take the pedals off and might have to take the handlebars off, but that is it. Wheels stay on.
Here is the Amtrak bike page http://www.amtrak.com/bring-your-bicycle-onboard
There is a new company out of Oregon City (a stop on the Cascades Amtrak Train just south of Portland). The company is called the Bike Concierage Thom Batty is the owner. email@example.com
503 314-6095. www.thebikeconcierge.com
He might be able to pick up the bike and you to shuttle you back to Portland. I have worked with him a little bit and he is very organized and helpful.
Please do let me know what you decide to do about getting the tandem back to Portland. I get asked this question quite a bit.
Oregon State Parks and Recreation Dept.
Bikeways Program Coordinator