The trackage is a seperate deal with the state, which I think is good until about 2025.
What they have to vacate is the former Portland Company Complex, part of which houses their museum. The property has been sold, but the new owners/developers have allowed them to stay so far, extending their lease. It is great for both the MNGRR and the City of Portland for them to continue, because they have been a fixture on the waterfront for 25 years this year.