Until June 2, 2011, Amtrak is offering $25 fares one-way for travel July 12-28, 2011 on their routes between NYC to/from Philadelphia, PA and Wilmington, DE. Click here to book this great deal.
Fares are limited and will sell out soon. The terms & conditions for these fares state that once ticketed fares are non-refundable, however, exchanges are permitted prior to the original travel date. In my experience, I have been able to cancel a ticket that was purchased online, since you can “ticket” your ticket the day of travel at the Amtrak station. The general change/cancellation policy is listed here.