220 ILCS 5/9-251
The Commission shall have the power to investigate all existing or proposed interstate rates or other charges, and classifications, and all rules and practices in relation thereto, of any public utility, where any act in relation thereto shall take place within this State; and when the same are, in the opinion of the Commission, excessive or discriminatory or in violation of any Act of Congress, the Commission may apply by petition or otherwise to any court of competent jurisdiction for relief.
The Commission shall also have the power, after a hearing had upon its own motion or upon complaint, to order any public utilities to establish and fix reasonable and sufficient joint rates or other charges or classifications. In case such public utilities do not agree upon the division between them of such joint rates or other charges the Commission shall, after hearing, establish such division by supplemental order.
220 ILCS 5/9-251