The last time I was there (a few months ago), the interchange track at the NW quadrant of the crossing at Yarmouth Jct. was still in place, possibly to connect to the existing Pan Am track to Portland. If this were the choice for passenger service from Lewiston to Portland (probably several years off, in any event), it's possible that the Yarmouth Jct. - Portland SLR segment could be abandoned or simply railbanked by the state, in which case it could become available to MNG (if MNG were interested). A while ago, Portland Trails expressed an interest in using the Back Cove swing span, and they presumably would also be interested in an abandoned ROW extending toward Falmouth.
On the other hand, I suppose the state could decide to use the SLR segment for Lewiston - Portland passenger service terminating at East Deering. This, like the current Amtrak station on outer Congress St., would require some kind of shuttle service to get people down town. I'm not sure that the possibility of a new station west of the Portland - South Portland bridge (mentioned by James) is compatible with the more recently publicized container terminal and Sprague boat yard proposed for that location.