65 ILCS 5/11-58-6
If at any time during the making of such grade separations, it appears to the satisfaction of the corporate authorities of the municipality, that the total sum of the tax authorized by the specified ordinance to be levied and collected will be insufficient to pay the proportion of the costs lawfully imposed upon the municipality, of those grade separations, the corporate authorities have the power, by ordinance, to set forth the total sum of the estimated deficiency and to provide for levying and collecting a direct annual tax, for not exceeding 5 successive years, sufficient to create a fund to pay, in annual installments, the estimated deficiency. However, this supplemental tax shall not exceed in any one year the rate of 1.25% of the value, as equalized or assessed by the Department of Revenue, of all the taxable property in the municipality.
The ordinance levying and collecting this supplemental tax, except as provided in this Section, shall be subject to all the conditions and limitations imposed by this Division 58 upon any original ordinance levying and collecting a grade separation tax. Before this supplemental tax shall be authorized, the supplemental ordinance shall be submitted to and approved by the electors of the municipality in the manner provided for in Section 11-58-3, unless the supplemental ordinance has been heretofore submitted to and approved by the electors of the municipality in the manner provided for in Section 2 of "An Act to enable cities, villages and incorporated towns having less than two hundred thousand inhabitants, to provide for defraying whatever portion may be imposed upon them by law of the costs and expenses of separation of the grades of railroads and of public streets and public places, and to provide for a direct annual tax therefor," approved June 17, 1929, as amended.
65 ILCS 5/11-58-6