Airline ticket prices are all over the place. Even tools like Farecast.com are at most educated guesses at whether prices will go up or down. Nothing's a worse feeling than buying a ticket at $300 only to see it drop to $175 three days later.

Amtrak, however, is much more predictable. There is only one direction train fares can go: up. Amtrak works on a strict bucket system. There are (I believe) four fare buckets. 330 days out, when a ticket is first bookable, the fares start at the lowest bucket. After a certain number of seats are booked (say, 25%, though I'm not sure of the specific percentage), it jumps to the next fare bucket. When 50% are booked, you get the third bucket, and the last 25% of the seats go at the highest bucket.

There is no such thing as fares going down, because unless a rush of people cancel their tickets, buckets will only fill up and make the fares go higher. Amtrak does not do what the airlines do and manually restrict certain buckets only to release them later and/or file new, lower fares. Perhaps Amtrak could make more money by implementing a system more like the airlines (so that they could, say, open only the highest fare bucket on all trains operating at Thanksgiving, their busiest time of year, since by definition 75% of the seats on any given sold-out train are NOT going for top dollar), but until they do, it is ALWAYS and 100% in your favor to book earlier rather than later.

That, combined with the fact that your tickets are fully refundable until you pick up the paper ticket stock from a station/machine, mean that it's a no-brainer to book now.