One fact I use to determine whether to pull the trigger on a trip or not is the number of rooms available. Since you can reserve up to 8 passengers, I enter 8 with the maximum number of roomettes it will allow, up to 8. I figure if 8 rooms are available out of a limited number, such as 17 on the off season Seattle Builder (13 in the single sleeper, 4 in the transdorm) , or 22 on the New York section of the Lake Shore (11 for sale each of two Viewliners), chances of them reallocating some into lower buckets are pretty good. If very limited, not so much, and I may want to go ahead and purchase.
In evaluating purchasing on my November NYP-EVR trip, I discovered some interesting behavior on how the website handles multiple room requests. On the Lake Shore, it would allow me to book individual rooms up to 5 rooms. On the Builder, however, it limited me to 2 rooms for 2 passengers, but it would allow 4 rooms for 8 passengers, though I couldn't book 4 rooms for 4 passengers. It would not let me increase the room count above 2 for 4 passengers. This interested me and I wanted to find the pattern this represented, since different behavior simply by train seem quite unlikely.
I discovered the following through a series of reservations requests on multiple days, confirming what I thought I saw for my initial request:
1. The website will not allow increases to the room count if the room count would require using inventory in a higher bucket.
2. When the number of rooms requested at double occupancy crosses a bucket jump, all rooms will at the higher bucket price.
The EB had low bucket of 619 for one passenger. I could book two passengers into two rooms, both at 619. but not three passengers into three roomettes. I could book 6 or 8 passengers into 4 roomettes, but at a cost of $1021 per room, which works out to the next bucket up, 684 (1021-337 single rail fare = 684). I concluded that there were 2 roomettes in the 619 bucket, and 2 in the 684 bucket.
The LSL had at least 5 rooms available, all at what I now know to be the $698 mid bucket. I could book 5 passengers into 5 roomettes without a problem.
I confirmed this behavior on several other departures on several other dates. It is consistent.
The good news is you can find some bucket jumps on a given departure. The bad news is since you are limited to 8 passengers on the website, so you cannot check if there up to 8 rooms available if there is a bucket jump in there.
I use room availability as part of deciding if it is time to pull the trigger for purchase or to wait. It is also useful in that, if you find a bucket jump and have a party size that requires multiple rooms, finding where the bucket jump is would let you split the reservation into two and get some of the rooms at the lower bucket rather than all of them at the higher bucket.
I did this as price and availability research, never intending to book multiple passenger reservations. I am sure if you wanted to book a party of multiple passengers into individual rooms across a bucket jump an agent could do it even if the website doesn't allow it. I'd think everything would go to the higher bucket, though.
I know there is a pretty large contingent that advocates just booking when you decide on a trip, whatever the current open bucket, then monitoring and requesting a refund if a bucket drop is discovered. I don't do this, the main reason is, since Amtrak is increasingly following airline style practices, that policy could well disappear unannounced. I'd hate to find a bucket drop on a booked reservation, request a modification to the lower price, only to find that they don't do that any more. Other reasons for my not doing it is I don't want to front the money/points, not inconsiderable at current sleeper fares, and then hope for a partial refund sometime in the future. Plus, not all Amtrak agents are created equal, not all agents know how to modify a reservation without potentially creating an artificial bucket jump by just requesting another room out of inventory and thereby holding two. I am also picky about roomette location and if I have one I like, I don't trust all agents to be able to hold that steady through a modification.
Website behavior changes as Amtrak IT tries to "improve" it, so I don't know how long this pattern will hold, but it's what it's doing now.