Do they have a published award chart?
Not exactly. They have the info on line, but you have to click through mishegas to get what you need.
Any train within the northeast corridor is 4k for coach and 8k for acela.
Obviously, as it gets further the value goes up, but you will maybe start considering flying. I think bos-phi is a good train distance. The acela is 5 hours, and a plane is about 75 mins. Add in differences in waiting time, parking costs (I street parked for free across the street from back bay station), comfort, and it gets reasonably worth it.
Of course if there had been avios availability that would have been better. But there wasn't. Then I could have done 4.5k each way flying.