As far as the Cascades is concerned, so far it appears folks are opting for Business Class. Surely some might do Coach as well, but I think it's worth the slight splurge particularly at lunchtime which in the past has brought some perks... at very least a free soft drink.
A helpful hint: I found that a little bit of money was saved by booking 513 SEA-PDX and 11 PDX-LAX on the same ticket as opposed to booking them separately. Since the two trains are a day apart there is no danger of missing this connection.