The State of Maine now owns this line, formerly known as the Lewiston Lower Branch (during MEC days) from Brunswick to Lisbon. I'm not sure exactly where their ownership ends and Guilfords resumes, but Maine owns at least to the Knight-Celotex mill in Lisbon Falls. I believe the exact milepost can be found in STB documents, but I cannot recall exactly.
The State of Maine started working on rehabilitating this branchline a few years ago, and currently the work is being done in earnest because actual service is anticipated to start up as soon as next month. The most likely customer at this time is the K-C mill (it has had several owners over the years, including Georgia Pacific) in Lisbon Falls. They make some kind of a particle board out of pressed woodchips. According to one reliable source, Guilford has had discussions with K-C about traffic, so it appears they will be the operator, even though Maine Eastern is the State's designated operator of its other state-held lines. Currently, Maine Eastern equipment is being used to spread new ballast on the line, and reportedly their tamper will be used.
I do not know what the State calls this line currently, nor what Guilford calls their segment (which is out of service and in horrible shape, I know because it runs past my house in Lisbon Village). I will update things as I find out more.