Adding:

I would recommend the train in this instance.

My breakpoint is one hour. If the train can get me somewhere within an hour of the plane, I take the train. Amtrak has not quite made it for NYC-Boston (I can do it door-to-door with the DL shuttle in 2.5-3.0 hours, sometimes even 2.0 hours. Acela -- including to/from Penn Station -- takes 4.0 hours).