how to get from seattle to vancouver bc when the Train is SOLD out?

Greyhound.

When a bus is full they add an extra.

Greyhound has a frequent schedule to/from Seattle & Vancouver, BC

What pjalittle said: www.quickcoach.com.

It's faster than Greyhound, as it makes fewer stops. The busses are much nicer, as well. It's usually about the same price, maybe a few bucks higher if you book later.

But, it's worth it. The problem with the Greyhound trip is that it stops in Coquitlam, and it takes forever to get downtown at certain times, due to traffic.

So, I'd highly recommend Quick Shuttle.

You can get a greyhound bus for about $30 per person. That said, you can usually rent a vehicle for a day for less than $50. It might be a better value to rent a car, because it's not much more expensive (with gas) and you can make your own stops for bathroom breaks, meals, etc.

There is an express Bus between

Seatac and Vancouver Airport.

There is a regular bus service to/from Vancouver. See schedules and rates:

http://www.quickcoach.com/schedule.htm

take another flight to van

chartered bus

only a 3 hour drive to vancouver, rent a car

Go on Craig's list and ask if anyone is going down to Seattle when you want to go.

Or, rent a car

Or, go Greyhound

Go by ferry, by plane, by Greyhound bus, or rent a car and drive.

Grey hound bus. There is bus service from seatac to down town