RRs are private companies, so when they owned the RoW, they paid property tax on the land and structures like bridges, stations etc. As RRs declined post-WWI, they disposed of unused RoWs. In the 1970s, many states started to buy RoWs as they were abandoned, considering they would be very hard to re-establish as NIMBY-ism got a voice in law. Most of the Twin State sadness has its roots in NH and VT buying the RoW that Guilford abandoned, then signing ill-considered contracts with an operator they hoped would benefit the area's economy. State-owned, so no property tax.
The Central Mass to South Sudbury was bought from the B&M by the Commonwealth in the 1977 transaction IIRC. The Amherst and Ware River remnants were picked up from Guilford later, but I don't know dates. I assume the CM RoW that paralleled the CV and B&A that was replaced by trackage rights in the 1930s mostly went into private ownership. The CM in between is going to keep lawyers busy for a while, and may have parts whose current owners won't give up. This is why Salisbury's 'Ghost Trail' and Amesbury's rail trail don't connect: one landowner on Elm St. bought a piece from the B&M in the '70s. The Newburyport City Railroad trail was also blocked for years by a landowner on High St. by March's Hill who'd bought the bit of RoW next to a house.