Yes, it's definitely more than gas prices. We live on one of the IL lines, and have watched the ridership balloon since roughly last September, well before the latest run-up in gas. The holiday trains were packed to the gills.
Actually, the numbers could be even better, but Amtrak doesn't have the equipment to spare. We are seeing 2 and 3 café cars on the Illini and Saluki just for the extra seating, and as you probably know the cafés don't really add that much seating. Since my corporate office is in Chicago, I occasionally check for seats between here and there, and if I don't plan far enough ahead it's impossible to get a seat in business class.
Amtrak travel generally is up, up, up. We had a chance to take a short Pullman trip last month, and the First Class lounge in Chicago was crowded to capacity, with barely a seat to be found. We were flabbergasted that coach was sold out on our (very long) train, the Lake Shore. The teeming throngs on the platform waiting to embark were astounding.