Amtrak and plane tickets are priced differently. For each class of ticket, Amtrak has X seats at price A, Y seats at price B > A, Z seats at price C > B, etc. When you check prices or buy a ticket, it will offer you the cheapest remaining ticket in that class. If you book Amtrak early, you'll find it is significantly cheaper than a plane ticket. Wait until a week until departure, and usually the airplane will cost less.