(a) General rule.--It shall be the duty of every contract carrier by motor vehicle to reduce to writing and file with the commission all contracts, or copies thereof, pertaining to the service of such carrier, and such schedules or other information pertaining to the rates of such carrier, in such form and detail, and at such times, as the commission may require. No such contract carrier shall engage in the transportation of passengers or property, unless the minimum charges for such transportation by such carrier have been filed with the commission, or copies of all contracts reduced to writing and filed with the commission. No reduction shall be made in any charge either directly or by means of any change in any rule, regulation or practice affecting such charge, except after 60 days notice of the proposed change filed in such form and manner as the commission may by regulation prescribe, but the commission may, in its discretion, allow such change upon less notice. Such notice shall plainly state the change proposed to be made and the time when such change will become effective. No such carrier shall demand, charge, or collect a less compensation for such transportation than the charges filed in accordance with this section, as affected by any rule, regulation, or practice so filed, or as prescribed by the commission from time to time, and it shall be unlawful for any such carrier, by the furnishing of special service, facilities, or privileges, or by any other device whatsoever, to charge, accept or receive less than the minimum charge so filed or prescribed.

(b) Reduced charges.--Whenever any such contract carrier shall file with the commission any schedule or contract stating a reduced charge for the transportation of passengers or property directly or by means of any rule, regulation or practice, the commission is hereby authorized and empowered, upon complaint, or upon its own motion, at once and if it so orders, without answer or other formal pleading, but upon reasonable notice, to enter upon a hearing concerning the reasonableness and justness of such charge, rule, regulation, or practice; and pending such hearing and 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, or defer the use of such charge, rule, regulation or practice for a period of 90 days; and 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 180 days beyond the time when it would otherwise become effective; and after hearing, whether completed before or after the charge, rule, regulation, or practice becomes effective, the commission may make such order with reference thereto, as would be proper in a proceeding instituted after it had become effective.