Take Acela Express, timing is 2:15 or so, and it's a nice ride.

Do NOT take the regular train unless you have all day to kill. AMTRAK does do a good job of keeping the Acela's on time, but doesn't really expend effort on the regular (much cheaper) service. I've been "Delayed Indefinitley" and faced with taking the next unreserved train with hordes of other abandoned passengers.

Acela is 100% reserved.