I was tempted to do this once on the Carolinian WAS-RGH. Business Class (full car) was jammed, and I was stuck in an aisle seat (assigned on platform by attendant) with essentially no view. Business was at the back of train (behind cafe), but coaches ahead of the cafe looked pretty full, and I think they were assigning people to cars (and perhaps seats) by destination, so I probably would not have been allowed to stay regardless.

Even stranger was during a late lunch, I tried to linger in the nearly empty cafe car, but the business attendant (seemed to spend a lot of time in the cafe car) and the cafe attendant were working hard to drive me back to my regular seat (tried to clear my trash before I was completely done, refused my attempt to buy a beverage so I could linger at the table, etc.).

One of those moments that leaves even someone who likes train travel thinking, "why didn't I just fly WN?"