§ 36-a. Municipal operation; deficiencies. If the board of transportation shall undertake or engage in public or municipal operation of any railroad pursuant to the authority of this chapter, the city shall provide for the payment of any expenses of operation, maintenance, interest, principal payments on indebtedness, contribution to sinking funds, and reserves on account of depreciation, to the extent and in the amount not provided for by the revenues derived from such operation. For such purposes such city shall, with respect to any such expenses heretofore or hereafter not provided for by revenues derived from operation, use money raised by taxation. Nothing in this section contained shall be construed to imply that the powers herein defined have not heretofore been vested in such city by law.