Mass. Gen. Laws ch. 161 § 120
A company accepting said sections shall file with its acceptance a schedule of nine different grades of fare and of transfer privileges. One such grade it shall designate as the initial grade which it desires to establish and which will, in its opinion, enable it to earn income sufficient to meet the cost of service. Four of the other grades shall provide for progressive increases and four for progressive decreases in revenue, and each interval of increase or decrease shall be calculated to increase or decrease the reserve fund by thirty per cent of the normal reserve fund. Within thirty days thereafter, after notice and a public hearing, the department shall either approve the schedule so filed or shall establish a schedule similarly designed in place thereof; provided, that if, during any period of sixty days, more than one company shall file its acceptance of said sections and a schedule of proposed grades of fare, the department shall not be required to approve or establish the grades of fare for any such company until thirty days after it has passed upon all schedules previously filed in accordance with this section. Whenever, by reason of any change in the existing rate of fare, there are less than four grades either above or below the rate of fare then in force, the company shall forthwith file with the department a schedule of additional grades of fare, so that there will always be both above and below the existing rate of fare not less than four grades of fare, and the department shall thereupon, within sixty days thereafter, either approve the schedule so filed or itself establish in lieu thereof the necessary additional grades of fare.
If at any time it shall appear to be in the interest of the public or of the company that the said schedule should be changed either with regard to the method or basis upon which the fares and transfer privileges are established, or because the steps between the different grades are too small or too great, or for any other reason, the company, with the approval of the department, may change the said schedule.
Except as above provided, the department may modify such schedule only after it has been in effect for a period of one year; provided, that no modification of the schedule which diminishes the rate of return on the stock investment shall be continued in effect for a period exceeding four months.
Mass. Gen. Laws ch. 161, § 120