When the MBTA started running trains to Providence, the Boston and Maine was managing the operation.. That explains the B&M SW. Conrail was still running freight service to Rhode Island but would soon defer to the Providence and Worcester.
The 70's and 80's were a transition period for the MBTA characterized by an eclectic mix of equipment including Budd RDC's, ex-New Haven stainless steel coaches, and a mix of motive power acquired from various railroads.