The land above the tunnel west of Wilson Street is owned by the Metropolitan Water District, as are several parcels between there and West Berlin. The ROW between Wilson and West Ledge does not have any listed owner on either town or state assessor maps, as does much of the ROW in Berlin. (East of Berlin is unambiguously owned by the MBTA per the 1976 sale.)
Most surrounding parcels (mostly swamp) are owned by the towns. However, some state-owned parcels (like 495) are also not listed on the assessor maps. So it's a toss-up whether the towns or the state effectively own the ROW. When a younger and less cautious EGE walked into the tunnel some years back, both local cops and a statie from the reservoir showed up. (Upon finding out that I was a clueless railfan rather than a teenager with a spray can, they ran my license for warrants and told me to be more careful.)
West of the reservoir, ownership is similarly complicated. Some town parcels, some DCR, some larger stretches in private hands, and many bits chopped up for subdivisions. The sections converted to trail are mostly DCR-owned.