65 ILCS 85/5
Rates charged for service and the use of such utility shall be sufficient at all times to pay the cost of operation and maintenance, provide an adequate depreciation fund and pay the principal of and interest upon all revenue bonds which by their terms are payable solely from the revenues derived from the operation of such utility. Any holder of a bond or bonds, or of any of the coupons of any bond or bonds of a city, village or incorporated town issued under the provisions of this Act may in any civil action, mandamus, injunction or other proceeding, enforce and compel performance of all duties required by this Act, including the maintaining and collecting of sufficient rates for that purpose and the application of income and revenue thereof.
65 ILCS 85/5