I say check out the Rail Sale page. They usually have great deals (70-90% off), but it is for limited routes and limited dates. Keep watching and you may get a good deal.
One year they had kids ride free and I took my 2 girls to WDW on the Amtrak. All 3 of us went for less than $200 (you had to pay regular price adult ticket to get the kids free.)
Maggie