Train is actually slower than a bus! But you will see some spectacular views during the train ride. The railway goes occasionally very close to the ocean and the views are really nice, especially in a setting sun.
Amtrak trains leave from Pacific Central Station. You should be there about an hour before the departure to go through the customs. Don't take any fresh fruits or vegetables with you. Our carrots got confiscated. The luggage is taken from you and you get it back at your destination (same as when flying).
On a train there is a bar where you can have some light meals, snacks and drinks. We didn't use the services so I can't comment about the options and quality of food on board.
When you reach to the US border then train is stopped shortly, the border control comes on a train and checks quickly the passports of all the passengers. This goes really quickly, about 10-15 minutes.
Actually during our trip the train broke down and we reached to Seattle an hour later than planned. Otherwise the train ride was nice and pleasant experience.