Someone mentioned it in another thread but mudslides along the route sometimes close the track, in which case you'll end up being bussed.

I wouldn't be too keen on taking the train as it's subject to delays and being shunted off to the side for BNSF (Burlington Northern Santa Fe, the company who owns the tracks) freight trains especially when the days are short. The train leaves VAC (Amtrak code for Vancouver, Canada) at 6p and it'll be dark so you're not going to see much. When you get to Seattle, it's a King Street which would require a transfer to SEA which isn't exactly close by.

A rental car would run about $100 including taxes and fees, and is generally available only from airport and downtown locations (don't rent at the airport to save on the airport surcharge) of National, Hertz, Avis and Budget.

You can take the Quick Shuttle bus to SEA for about $52.25 OW and the busses do have wifi onboard. See: quickcoach.com