Adding my 2% of $ 0.01 to the discussion and going back more than a half-century, one of the reasons for the initial success of RDCs on the B&M was maximized equipment utilization. A string would cover multiple routes, not being dedicated to -- for example -- a Rockport-Boston turn. When intercity routes were eliminated, this permitted even greater focus on containing commuter expenses. Aging and deferred Budd maintenance would lead to loco-powered push-pull operation, but maximized equipment utilization remained/remains SOP into the T era, regardless of the contract operator.
Until and unless under-harbor north-south commutes start happening and/or until and unless major anti-pollution regulations are in effect, requiring electric locomotives, the likelihood of the T sacrificing maximized equipment utilization by having route-specific equipment seems less than likely.
As a footnote, there is a probably-late-50s B&M employee magazine with a line-chart showing the routing of one Budd set throughout a typical weekday day. That set was busy.
"A gray crossover is definitely not company transportation."