...I have a few questions. I will have to assemble my bike for the trip from the Seattle station, however I seem to recall the Cascade will allow me to roll it on for the trip to Vancouver. Is this correct? Also how is the neighborhood around the Seattle station. I would probably spend the night at a HiSeattle hostel.
Yep, Amtrak Cascades allow roll-on service, so no need to box. In fact in most cases they don't like boxed bikes on these trains, from what I've seen.
As for the neighborhood around King Street Station is slightly sketchy at night, especially going over towards the hostel. During the day when it's full of people, not so much. I've never felt threatened, though, but some others may get weirded out. Thankfully the American Hotel (HI-Seattle) is a few short blocks from the train station.
One thing to note about the hostel is they offer secure bike storage in the basement, so no need to lock it outside overnight.