The governing body in any city operating a street-railway system or bus system shall levy annually at the time when other taxes are levied a sufficient tax to pay for the necessary costs and expenses of operating said system and for the purchasing of necessary equipment and improvements therefor only in the event the revenue and income derived from such system are insufficient for such purposes and to pay a portion of the principal and interest on bonds issued by such city under the authority of K.S.A. 12-1774, and amendments thereto. Such levies as are authorized herein shall be in addition to all other levies authorized or limited by law.
K.S.A. 12-803b