Mass. Gen. Laws ch. 161, § 154

The department may exclude from a transportation area any city or town, but such exclusion shall not prevent the operation of street railways in or through its territory. The department may also exclude any city or town or part thereof, the inclusion of which would, in its opinion, handicap the efficient operation of a transportation area. The department may permit the operation of street railways without the commonwealth by any transportation area which borders on the boundary line of another state.