Probably not much slower either, with the ground delays you often get going into the New York City area. Amtrak is on option on that route as well, again nicer, bigger seats than Coach class onan airline.
Actually it amazes me so many people travel by airline between Boston, NYC and Washington DC when Amtrak offers a much nicer product that probably isn't any slower once you factor in security lines. Recently United had ads bragging about how close EWR is to Manhattan but Penn Station is right in Manhattan.
I feel bad for the passengers I occasionally bring from IAD to EWR. In the afternoon theres almost always a Ground Delay and trains and intercity buses usually have much better "packs" than the Dash 8.