Section 125. In order that the department may at all times be in a position to take such action as the public interest requires, the companies shall furnish it each month with such statements as it requires, showing the condition of the reserve fund, the income and expenditures of the previous month and such other information as the department may desire. The commonwealth shall be divided by the department into street railway districts, and for each district within which one or more companies has accepted said sections the department shall appoint one or more resident supervisors for terms of three years, their salaries and expense allowance to be fixed by the department and paid by the company as an operating expense, or, if their duties extend to more than one company, their salaries and expenses shall be apportioned among the several companies in respect to which they act, in such manner as the department may determine. Said supervisors shall keep in constant touch with the operation of the companies and inform the department of all complaints and criticism of the service rendered.

If the department deems any special investigation of any company necessary it may order the same at the expense of the company.

The department may require such changes in the management and operation of any company which has accepted said sections as it deems necessary for the efficient conduct of the business of the company in the public interest.

Any foreign company furnishing electric light or power to any street railway company operating under said sections shall file annually with the department a schedule of all rates charged to all its customers for power where the electricity is furnished by a central plant to others than the said railway company and with such other information in such form as the department requires. The department may prohibit the transmission of electricity for either light or power unless such information is filed as requested and the prices charged therefor are determined by it to be fair and reasonable.