Unlike the airlines, Amtrak doesn't play complex fare games that are just about impossible to figure out. The deal is simple: as the train fills up, prices go up. To be more specific, there are a number of price levels, or "buckets." Coach and each room category (roomettes, bedrooms, and family bedrooms) have their own buckets. The reason why the other passengers you spoke to paid more is almost certainly because you got your tickets before they did. To answer your question, both booking early and being flexible with dates is important for getting the lowest price. The price difference between two adjacent dates can be quite dramatic!
