Who's Booked Business Class and Ridden in Coach?
I have, a few times. No, Amtrak didn't downgrade me.
Non-Acela business class off the corridor involves one car, or part of a car. Oftentimes most seats are reserved and you must share a seat pair with someone else, most likely sitting in an aisle seat and worst case, where there's no window or only part of a window. Meanwhile, in coach there are loads of available seats.
Is it a bummer to pay for business class and end up sitting in coach? It depends. Biz class upcharges are often small and may even be covered by a voucher. You get a newspaper, snack and on the Pacific Surfliner, a small bottle of wine or perhaps beer. Just take these items next door. Coach seats are plenty spacious, and if you're lucky the adjacent coach car will have lots of available doubles. So I'm not necessarily bummed.
It's just kind of weird.