Section 23. The commonwealth, acting by and through the executive office for administration and finance, may enter into contract or contracts with the authorities created pursuant to this chapter providing that fifty per cent of the net cost of service of each authority shall be paid by the commonwealth, and shall not be assessed upon the cities and towns constituting the authorities established by section two and section three. Such amount, not to be so assessed shall be called contract assistance.

Contracts shall provide for payment of debt service by the commonwealth when due except to the extent that the authority shall have previously notified the state treasurer that the revenues of the authority are sufficient for the purpose.

Any debt service on bonds issued by an authority, for which contract assistance is provided, shall mature serially beginning not later than ten years after the date of issue and ending not later than forty years after the date of the bonds, so that the amounts payable in the several years for principal and interest combined shall be as nearly equal as in the opinion of the authority as is practicable to make them or, in the alternative, in accordance with a schedule providing a more rapid amortization of principal.

Any contracts or agreements made between an authority and any private company or carrier for which contract assistance is provided shall be subject to the following limitations: (i) in determining whether assistance is needed under this paragraph with respect to an operating agreement with a private transportation company, and in determining the terms of such assistance, the authority shall review the entire transportation operations of the company and its affiliates and shall make a finding that the assistance will not permit the applicant company to make more than a reasonable return overall; and (ii) that the assistance shall cover only those services determined by the authority to be in the public interest.

Any contract under this section shall include such provisions as the secretary deems necessary and desirable to assure the efficient operation of the authority, and the minimum burden on the commonwealth and on the cities and towns within the authority, and to insure contract assistance is provided for projects which are consistent with the program for public mass transportation of the authority.