The form for all reports required under sections 137.975 to 137.985 shall be prescribed and furnished by the commission. Every railroad and street railway company operating in this state shall file annually, on or before the thirty-first day of May, a true and accurate statement of the main line track mileage of such company in operation in each county of this state on the thirty-first day of December of the preceding year, together with the total mileage of each car company for such year. Each private car company shall file annually, on or before the thirty-first day of May, a report showing the total car mileage from railroad and street railway companies, the dates and original cost of acquisitions and other information the commission may require for the purpose of carrying out its duties under sections 137.975 to 137.985.