Section 34A. (a) The department shall have general supervision of every affiliated company, as hereinafter defined, with respect to all relations, transactions and dealings, direct or indirect, with the carrier with which it is affiliated which affect the operations of such carrier, and shall make all necessary examination and inquiries and keep itself informed as to such relations, transactions and dealings as have a bearing upon the rates, financial condition and practices of such carrier. Such relations, transactions and dealings, including any payments by a carrier to such an affiliated company or by such an affiliated company to a carrier for property owned, leased or used by such carrier or such affiliated company for transportation purposes shall be subject to review and investigation by the department in any proceeding brought under this chapter or under chapter one hundred and fifty-nine A, one hundred and sixty or one hundred and sixty-one, and the department may order such affiliated company to be joined as a party respondent with such carrier in such a proceeding.

(b) Every affiliated company having such relations, transactions and dealings with the carrier with which it is affiliated shall make such annual or periodic reports, and in such form, as the department may by regulation prescribe, in order to give the department effective supervision over all such relations, transactions and dealings.

(c) Officers and employees of the department may be authorized by it to examine the books, contracts, records, documents and memoranda or the physical property of any affiliated company subject to this chapter or chapter one hundred and fifty-nine A with respect to any relations, transactions or dealings, direct or indirect, between such affiliated company and any company so subject, and, for any examination so authorized, shall be entitled to full access to the subject matter thereof. No such officer or employee shall divulge any fact or information coming to his knowledge during the course of such examination unless directed by the department or by the court, or authorized by law.

(d) For the purposes of this section, the words ''affiliated companies'' shall include any corporation, society, trust, association, partnership or individual (a) controlling a company subject to this chapter, or chapter one hundred and fifty-nine A, either directly, by ownership of a majority of its voting stock or of such minority thereof as to give it substantial control of such company, or indirectly, by ownership of such majority or minority of the voting stock of another corporation, society, trust or association so controlling such company; or (b) so controlled by a corporation, society, trust, association, partnership or individual controlling as aforesaid, directly or indirectly, the company subject to such chapter; or (c) standing in such a relation to a company subject to such chapter that there is an absence of equal bargaining power between the corporation, society, trust, association, partnership or individual and the company so subject, in respect to their dealings and transactions.

(e) Whenever, in any proceeding before the department under section fourteen, twenty, or twenty-four the reasonableness of any payment, charge, contract, or purchase, sale, obligation or other arrangement between a carrier and a company related to it as an affiliated company, as defined in paragraph (d), shall come into question, the burden of establishing and proving the reasonableness of such payment, charge contract, purchase, sale, obligation or other arrangement shall be upon such carrier.

(f) The supreme judicial court shall have jurisdiction in equity to enforce compliance with this section and with all orders of the department made under authority thereof.