The Riders Guide describes the difference on board between Coach and Business but these seem marginal. Are there any more...?
- A business class ticket includes a voucher for $3 credit in the bistro.
- Business class passengers may use the Metropolitan Lounge at Portland, Oregon station (the only lounge on the Cascades route). This offers free coffee and chilled water, and is both less crowded and better air conditioned than the main station hall. Boarding is straight from the lounge. However, the lounge opening hours do not coincide with all Cascades services (we could not access it for the southbound 9:45AM departure).
- Business class carriages are arranged 1-2 rather than 2-2; thus you have better chances at a window seat, and if travelling alone you can do so without a seatmate.
- There are fewer seats in business class carriages anyway, and in my experience the load factor is also lower. On my most recent Cascades trip, my wife and I were the only passengers in the carriage! This means a more peaceful experience, better ratio of luggage racks to passengers etc.
Business class may offer better earning in Amtrak rewards; I am unfamiliar with the details of that scheme as it is not available to non-Americans.
"Priority boarding and detraining" sounds like a small benefit -- it is anything, but. At least when going north into Vancouver, BC. You will be lining up for border/customs outside and if the line is long enough, then on the platform, under open sky as well (there's an awning but only so many people fit under that). So "priority detraining" in this direction is a massive benefit as it includes priority through the border. If you add "Fewer passengers in each car" compounded by the fact that not only there are fewer seats in the first place, even fewer will be occupied because people don't realize what I just described. So your chances of just hopping off the train and walking right up to the border official without much waiting is pretty good.
Going south I haven't been able to see any reason to waste money on a business ticket, though.