Any such city may expend moneys to pay part of the costs of operation of public transit systems if revenues of such system prove to be insufficient to cover all costs and for such purposes such cities may levy a tax each year upon all the taxable tangible property within such city for such purposes and to pay a portion of the principal and interest on bonds issued by such city under the authority of Kan. Stat. Ann. § 12-1774, and amendments thereto.