Whenever there shall be filed with the commission by any contract carrier any schedule or contract stating a reduced charge directly, or by means of any rule, regulation or practice, for the transportation of passengers in intrastate commerce, the commission is hereby authorized and empowered, upon complaint of interested parties or upon its own initiative at once and, if it so orders, without answer or other formal pleading by the interested party, but upon reasonable notice, to enter upon a hearing concerning the lawfulness of such charge, or such rule, regulation or practice, and pending such hearing and the decision thereon the commission, by filing with such schedule or contract and delivering to the carrier affected thereby, a statement in writing of its reasons for such suspension, may suspend the operation of such schedule or contract and defer the use of such charge, or such rule, regulation or practice, for a period of ninety (90) days. If the proceeding has not been concluded and a final order made within such period, the commission may, from time to time, extend the period of suspension, but not for a longer period in the aggregate than one hundred eighty (180) days beyond the time when it would otherwise go into effect. After hearing, whether completed before or after the charge, or rule, regulation or practice goes into effect, the commission may make such order with reference thereto as would be proper in proceeding instituted after it had become effective. If the proceeding has not been concluded and an order made within the period of suspension, the proposed change in any charge or rule, regulation or practice shall go into effect at the end of such period. The carrier may voluntarily suspend such schedule, rule, regulation or practice for further periods beyond the one hundred eighty (180) days and until the proceeding be concluded.