Thing is I've read that it's usual best to book Amtrak tickets as soon as they are available. I've also seen when doing to search for Feb next year which is 11 months out the cheapest fairs are already available so if it's the lowest fair I would not think they are going cheaper?
That really depends on the route. There are some Amtrak (or affiliated) routes that effectively aren't "reserved". They call them "unreserved trains" even though you can buy a ticket that specifies a date and train number. They will theoretically use advance ticket sales to determine how many cars to place in service. The fares don't change on these routes except for fare sales and various discount codes. An unused ticket can theoretically be used any time on the day specified on the ticket up to a year afterwards. One also has the flexibility to use a monthly or multi-ride pass on these routes. They will also sell tickets on board the train, although there's a surcharge if there was means to buy a ticket at the origin station. I've never heard of such a train "selling out" and I don't believe it's normally possible.
If you're using a rail pass segment, it doesn't really matter what the retail fare is. However, they have limits on the number of tickets that can be obtained via the pass.
Reservations for train travel should be made as far in advance as possible; seats available for USA Rail Pass passengers are limited on each train. We do not recommend waiting until the day of departure to make your reservations since there is a greater chance that seats allocated for the USA Rail Pass may not be available on your desired train. If your plans are not flexible, non-USA Rail Pass seats may be available at an additional cost.