In the past, Amtrak definitely would overbook coach. Not by a ton, but maybe a few percent. During the early COVID days, I understand that was stopped simply because they had to cut back to ~50% or so of seated capacity. Now that they no longer have those restrictions, I don’t know if they have returned to slight overbooking, or if they cap it at 100%. If those who say Amtrak doesn’t overbook have current info, then fine. But if they’re speaking from the basis of guesses/assumptions, I’d be wary of that.
I can’t say that every train would get overbooked, and premium accommodations definitely would be sold to the limit of planned capacity. It would actually surprise me if (COVID changes notwithstanding) they didn’t get more effective at overbooking, since they should have a better idea these days of how many no-shows there are on which segments of which routes, on which days of the week; how many late cancels/last minute changes they get, etc.
Selling only 100 tickets on a 100-seat train leaves a lot of potential revenue on the table when you know, for example, that 2% of passengers miss their train, and another 3% will rebook for a different train a few hours from departure.