They never bought that 3-mile track segment, so CSX owns it now.
I should have included the actual quotation. The article does mention the Gardner to Barbers purchase + trackage rights over the 3 miles, but it later says:
P&W also filled a couple of small but strategic gaps in its route map in 1983 when it bought the 3 miles of B&M trackage that connected the Gardner branch with the rest of the P&W system in Worcester, plus 1 mile of it at Gardner. Interestingly, this purchase required P&W to give up its stock in the Vermont & Massachusetts, a 19th century company that officially owned a portion of B&M's main line between Fitchburg and Greenfield, Mass.
That said, the article's statement could also be wrong or not digging into the nuance of the transaction & ownership.
Worth noting is that in CSX's STB filing for the purchase, it discusses the trackage rights agreements for NS to move intermodal and/or autoracks over the line, and makes the statement (in reference to the routing):
These trackage rights agreements will allow NSR, upon consummation of the Proposed Transaction, to move up to one train pair per day, carrying intermodal and automotive vehicles traffic, between NSR’s connection with CSXT near Voorheesville, NY, and the intermodal terminal located near Ayer, MA, over CSXT’s east-west rail line between Voorheesville, NY and Worcester, MA, then over P&W’s rail line between Worcester, MA and Barbers Station, MA, over Boston & Maine’s rail line between Barbers Station, MA and Harvard, MA, and finally over PAS’s rail line between Harvard and Ayer.