I don't see how 100% NS control of the PAS will be construed as anti competition, or antitrust. CSX got control of the B&A, as part of the Conrail split. Since MA, and CT have among the highest population density, there are millions of potential customers in Springfield, Worcester, Boston, Hartford, New Haven, and suburbs. Nobody in those places would object to having more transportation options. Right now, CSX is the only class one which is in New England. PAS is roughly comparable, and parallel to the B&A.
East of Ayer is another story. Who knows if Maine and New Hampshire have the traffic density to attract a class 1. Maybe NS and CSX would form a joint operation there, if T.M. ever decides to sell Pan Am, just like Conrail, and NYS&W.