Some of this echoes what johnpbarlow has already posted, and some is new.
What is critical to remember is the PC-Contract-Whenever-Possible approach of Fink 1.0 . In addition to the PW purchase of the Gardner Branch in 1974, the PW did get rights on the Worcester-Ayer line sometime in the middle-late 1980s. The PW even had its own "No Trespassing" signs at the Prescott Street, West Boylston, crossing and probably others. The PW also installed solar power for the Prescott Street protection, the town's municipal power stopping a few hundred yards before the crossing and welled batteries providing the pre-solar power. On the subject of rights, B&M and then Guilford maintained rights to Norton -- now Saint Gobain -- off the Gardner Branch.
When Guilford sought to downgrade West End service to increase savings on ROW, power, and crews, it began to shift racks and coal to Conrail via the Worcester gateway with the PW allowing overhead to Shea, Shea to Ayer reverting to Guilford. As part of the "upgrade" of the Worcester Main, Guilford got the West Boylston Municipal Lighting Plant to extend electrical service to the crossing, replacing the solar. How many other crossings in other municipalities were similarly affected is something I don't know.
Sometime before the middle of 1990, Guilford and Conrail attempted to run general freight over the PW to Shea in violation of the earlier rights agreement. PW refused to issue orders, and a negotiation took place that eventually permitted general in addition to coal and racks.
The breakup of Conrail and NS's interest in getting as much of a favorable rate division on the coal and racks that generally originated on its newly-acquired lines, not to mention general and IM, led to PAS and its not-yet fully-realized rehabilitation of the RJ/CPF-312 route.
If the pre-G&W PW felt a bit whipsawed by Guilford, the pre-and-post-G&W NECR dealings with Guilford/PAS have probably done little to reduce PW suspicion of PAR/S under the G&W. The Hill Yard barely has capacity as it is. Add racks and general to and from the PW -- absent at least 3 more tracks being rebuilt -- and capacity will be exceeded. Additionally, the Clinton siding is adequate only for locals and work trains. Increase the number of movements with racks to and from Worcester and the new chokepoint of power swaps at Greendale. The old auto yard might serve for storage, but given the need to get headway from the T, little else.
However wasteful and inefficient it might seem, the PW's continued use of the Gardner Branch provides an operational flexibility and certainty that shifting to the Worcester Main would not, absent a major PAR -- not PAS -- or Commonwealth investment in a major passing siding and increased capacity at Greendale and the Hill Yard.
"A gray crossover is definitely not company transportation."