Railroads all over the Country run over other railroads tracks to get to and from their own lines. Just to name a few, BNSF over the UP over Tehachapi, UP over BNSF over Cajon Pass, UP over BNSF to get to Seatle, the BNSF over UP over Donner, I could go on and on.
With better service which the NS would undoubtadly provide (how could it not?), traffic over the old B&M and MEC could go up considerably. Not only is there an amazing amount of rail type freight that is trucked to and from ST customers that they could win back, but NS would probably do alot better getting traffic off of the MMA and from the Maritimes. Unlike the ST, they would probably also be willing to invest in Wodchip and Log cars that could add some serious tonnage to the former Maine Central - tonnage that the MEC always handled but that the ST pissed away. The same could be said of Bunker C oil which the MEC handled alot of prior to the ST and which the ST pissed away by way of lousy service and in the case of IP, an accident at Jay in the 80's that caused an Oil spill that the ST never took responsibilty for.
The only down side of such a takeover would be a likley loss of tonnage moving over CSXT's Boston Line.