Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 22C, § 51

The colonel, upon the petition of a railroad corporation, or of a railway company, or of The Boston Terminal Corporation, or of Railway Express Agency, Inc., or of a common carrier of passengers by water for hire having a usual place of receiving or discharging passengers within the commonwealth, may from time to time appoint as special state police officers as many of the persons designated in said petition and being citizens of the United States as it may deem proper, for the purposes and with the powers hereinafter set forth. Any person, appointed as a special state police officer upon the petition of a railroad corporation, may act as a police officer upon the premises and vehicles of any corporation transporting passengers or property by motor vehicle under the joint control and management of said corporation and said railroad corporation, which, for the purposes of this and sections fifty-two and fifty-three, shall be considered as being the premises, cars and vehicles of said railroad corporation.