For pretty consistent high bucket Southward and Low bucket Northward fares the 18 December - 15 Jan period stands out. Snowbirds going South.
For consistent high bucket Northward and low bucket Southward fares the 28 March to 31 May period stands out. Snowbirds going North.
The only period of note with high bucket fares in both direction is the first 25 days of September. No idea why except perhaps being that far in the future the lack of demand hasn't yet had time to drive the fares down?
FWIW, almost all of the fare buckets for the Auto Train have recently risen by about 1% and the chart in Post #7 has been updated to reflect those small increases. I really can't tell about the other trains because there's a sale going on and I don't bother with discounted fares. It's hard enough to keep that chart updated when the original AmSnag... http://biketrain.net/amsnag/amSnag.php ...is out of commission.