Actually right now Amtrak is running a winter sale with some seats on the Auto Train going for $74 one way. No code is needed for this special, but travel must be between January 21st and April 5th. And they still charge for the car too. Kids are half price and it must be booked online.
Note: Fares are non-refundable, however the value of tickets may be exchanged for another trip on Amtrak.