35 ILCS 200/11-100
If any railroad company owns or uses operating property partly within and partly outside of this State, the Department shall determine the value of the entire operating property of the railroad but shall take only that part of the entire value as is represented by the average percentage of (a) the length of all track including main, second and additional main track, side track and turnouts within this State, (b) its gross revenues arising from railroad operations in this State, (c) the reproduction cost of its operating property within this State, as determined by the Interstate Commerce Commission of the United States, or other competent authority, plus additions and betterments, less retirements and depreciation. Nothing in this section shall be construed to preclude the use or substitution of other factors or methods as may appear reasonable and necessary in determining the proportion of a railroad's operating property within this State.
35 ILCS 200/11-100