I was waiting for a train at BWI at 11 pm last week, and an NS freight came down Track 2 (middle track, concrete ties etc.) at 50 mph. So there are no dedicated tracks for high speed trains. It's entirely dispatcher option as to which track to use for which train.

HOWEVER, maximum track speeds do vary. South of Philadelphia, where the railroad is three main tracks, one (usually the east track) has a speed limit of 80 mph, while the other two are good for 125 (or, for Acela Express, 135). North of Phila., where, there are four tracks, the two outside tracks have lower speed limits as far as Trenton. North of Trenton, where NJ Transit has paid for welded rail and concrete ties on both "local" tracks, I understand the maximum speed is the same on all four tracks.

With respect to freight, the only segment of the NEC with significant through freight is Perryville, MD to Baltimore (about 60 miles), with about 20 million gross tons of freight per mile per year. Limited through freight service also runs north from Perryville to Newark, DE. North of there, virtually all freight is local switchers only. Between Baltimore and Washington it's local freight only, except for the Pope's Creek and Chalk Point coal trains (which leave the NEC at Bowie, halfway between Balto and Wash).