The ROW that MNG uses was formerly the Portland Yard for Grand Trunk RR. GT's yard ended at Commercial Street, and there was (still is, buried) track down Commercial to connect to the various wharves and on the other end to the Maine Central. At the west end of GT's yard was a swing bridge, from there the track headed north and west to Canada. GT was always standard gauge.
GT's swing bridge had a fire during the mid-80s, as I recall, and was never repaired. The yard was pulled up, and acquired by the State of Maine (for railbanking, I presume). When MNG started up in the mid-90s, they received a ROW lease from the State for their track - this lease was recently renewed for another 20 years. Whether MNG had anything to do with it or not, at some point the City of Portland worked to revitalize the former yard, it's now got a nice walking/jogging/biking path with a lawn; the walking paths extend around the city now.