I have traveled from NH to Boston many times via Amtrak, never had anyone question the contents of my pack. You pretty much get on the train, find yourself a seat and wait a few minutes for the guy to take your ticket. As far as I could see, there were no security officials at Boston North Station, this might be different at different stations though. I carried a 70 liter Kelty with me on one of the rides and it fit perfectly in the overhead. There is plenty of room. You could also just put it in the seat next to you. Worst case scenario, you just put it between your legs.