Nothing in this Act shall be construed to confer upon the Authority the right, power, or duty to order or enforce the abandonment of any present property of the railroads or the use in substitution therefor of any property acquired for the railroads in the absence of a contract duly executed by the railroads and the Authority setting forth the terms and conditions upon which relocation of the right-of-way and physical facilities of the railroads is to be accomplished. No such contract shall be or become enforceable until the provisions of the contract have been approved or authorized by the Illinois Commerce Commission.