Section 116. The cost of the service shall include operating expenses, taxes, rentals, interest on all indebtedness as hereinafter defined, dividends on preferred stock, an interest return at the rate of six per cent per annum upon the stock investment as determined by the department under this section and such allowances for depreciation, obsolescence, and losses in respect to property sold, destroyed or abandoned as may be fixed from time to time in the case of each company by the department, and all other expenditures and charges which, under the classification of accounts of the interstate commerce commission and under the laws of the commonwealth, are properly chargeable against income or surplus. The words ”stock investment” as used in said sections shall mean the capital investment, as hereinafter defined, less the amount paid in for outstanding preferred stock, bonds and other evidences of funded indebtedness. The words ”capital investment” as used in said sections shall mean the amount of cash or fair value of the property paid in for stock, bonds and other evidences of funded indebtedness and properly expended for capital purposes, such amount to be determined by the department; provided, that if it has approved the issue of any such securities, no further determination in regard to the capital investment represented by such securities shall be necessary.