I don't believe any of the Eastern north of Newburyport is a rail-trail, but its proposed as one. As far as Seabrook is concerned, I'm not sure what the problem is. The NEC passes within close proximity to Millstone in CT.
Have the tracks been removed from Portsmouth south to Hampton yet?
Sure, hitting the destinations along the coast would be the ideal way (Hampton, Salisbury, etc), but there is currently an active track to Portsmouth (via the branch, that would need upgrading). Plus, you're looking at almost double track the entire way.
I guess its a moot point if NH doesn't want it. But the thing about Portsmouth/Seacoast service is that its more of a "two-way street" for NH... its not just for people to go to jobs in Boston, its for those in the Hub to hit up the Seacoast, which is more of a destination than Nashua/Manchester.