Right now, there are six codes to choose from.

The best of the bunch is code V822. It will knock fares down by 20% on all unreserved trains as far north as Albany, as far east as Boston, as far west as Harrisburg and as far south as Virginia. There are no blackout dates, and you can travel as late as Dec. 16 on this code. When you're picking your train on Amtrak's site, look for trains with a "U" symbol under the "Accommodations" column -- those are the unreserved trains. If V822 doesn't work, try V729, a very similar discount.

Want to try the high-speed Acela train? Use code V711 for Acela and Metroliner trains on weekends (except Sunday 1-6 pm) from now through June 30. It'll knock 20% off your fare. It won't work for trains from april 8-12 or May 28-June 1. If that code doesn't work, V745 and V704 are very similar discounts.

If you're traveling outside the Northeast Corridor, your code is H406. That'll knock 5% off almost all train fares nationwide for travel from now through August except on the Easter, Memorial Day and Fourth of July weekends. It doesn't apply to trains to Canada or to Northeast Corridor trains on Fridays and Sundays, but if you're traveling in the NEC, you can get a better deal with V822 anyway.