Taking the train Vancouver to Seattle
 

I'm in the final stages of planning a holiday in North America and was hoping to take the train from Vancouver to Seattle. On checking the Amtrak schedule, it appears that there are only two trains - one at 6.30 am, and the other late in the afternoon which arrives in Seattle at 10 pm. Amtrak use buses during the day. So which do we do? Take the late train, and be wandering around Seattle in the dark to find our hotel plus part of the trip would be after sunset so we wouldn't be seeing the views, or go by bus (not my favourite choice). I'm assuming VIA don't run a train on this route as I can't find anything on their website.
I've also been told that, as a visitor to North America, I can't buy tickets on the Amtrak website but need to go through a agent in my country.
Any information would be appreciated.

Take the train - it will be the Amtrak Cascades. We've done both morning and evening routes. Personally, I preferred the morning train - although you will need to get up early, and you need to plan to arrive at the station at least an hour or more early as you will need to go through Customs before boarding.

At night, don't count on getting in by 10p, best think 10:30p-11p more likely. There are tons of taxis waiting, crowds jostling for them, then you wait for the next round of cabs, etc.

Either way, try to get assigned seats (at the station as you check in) on the right hand side, as you will get the best ocean views. Shortly before the U.S. border is the best view for eagles - pay attention, and you'll see that those aren't all sea gulls, but tons of eagles are mixed in.

I cannot really help with your question about needing to buy tickets from an agent in your country, but here is Amtrak's FAQ page: http://www.amtrak.com/crossing-the-us-canadian-border