Starting with the last first: The Cascades (normally) operate with Talgo equipment, which does not have the large double-deck "Sightseer Lounge" found on the Superliner equipment used on most Western US Amtrak trains, nor does it have the classic "dome cars" still found on the Canadian operated by Canada's VIA Rail. However, the seats are comfortable and the windows are plenty large enough; you'll have great viewing opportunities. For the best views, try to secure a seat on the ocean (west) side. The Cascades do not have a full service dining car but there is a "Bistro" cafe car which serves hot and cold items; the clam chowder is worthwhile if you can secure a cup before they run out! Adjacent to the Bistro car there is a car with tables and seats where you can enjoy your purchase as you would in a diner, or you may take it back to your seat (seats have tray tables). There are standard US 110 volt power outlets at your seat, although a "power strip" with a short extension cord might be advisable as they are near floor level.
US Customs has an office at the station in Vancouver, so you will clear customs at the departure station before boarding. Allow extra time for that; 45 minutes or so should be sufficient. Also keep in mind that if you are checking luggage to Seattle the luggage must be checked in (after clearing Customs) 45 minutes prior to train departure. There is no fee for checked luggage on Amtrak (not yet, anyway...casts wary eye at current Amtrak management!); just allow the extra time. For Immigration clearance, normally you fill out your form on board and an Immigration officer rides the train and checks your credentials on board as you near the border. Business Class passengers get priority processing through Customs as well as larger (2+1 as opposed to 2+2) seats on the train; it may be worth it to upgrade just for that reason (it is to me!), especially going northbound. Northbound all processing is done upon arrival and coach class passengers must wait in their seats until business class passengers have been unloaded and processed. You didn't mention whether or not you are returning to Canada by train.
It's a nice little trip with a lot of scenic views; I've done it twice northbound and once southbound (second return was via ferry from Victoria BC). Have a great time!