Amtrak Informative - Amtrak Weekend Getaway Fares
Amtrak is now offering deep discount fares for weekend trains along the Washington-New York-Boston corridor. For example a one-way ticket from Washington to NY is $49. The regular fare is $70. This applies to Regional and Keystone service only. There are some restrictions such as the tickets are non-refundable.
Go to AMTRAK.COM and click on Hot Deals for the details.