Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 161A, § 4

(a) In addition to the powers granted to the authority under section 3 and all other powers granted by law, the authority shall have the power to establish on a self-liquidating basis one or more separate units of mass transportation facilities and equipment to furnish, in each instance, express service or local service between specified terminal points and over a fixed route or routes. In establishing such separate units, the authority may enter into one or more unit lease arrangements with such persons, firms and corporations as the authority shall select and franchise. Each such unit lease arrangement shall provide for the following:

(b) To meet the expenditures necessary in carrying out the provisions of this section, the authority may issue bonds in accordance with the provisions of clause (4) of the first paragraph of section 13, and such bonds shall provide, in addition to other provisions allowed under this chapter, that all payments of principal and interest shall be made solely from (1) the rental or lease charges received by the authority under its lease with the lessee of mass transportation facilities and equipment as aforesaid, which said lease may be assigned by the authority to secure the obligations of said bonds; or (2) in the event the authority terminates said lease from the income derived from operation of said mass transportation facilities and equipment; provided, that bonds issued for the purposes of this section shall not be included in the computation of the bonds to which the limitations on amount contained in section 13 shall apply.