Section 25. A company, for the purpose of building an extension, or acquiring land for pleasure resorts, or acquiring or building power houses or car houses or park buildings, or acquiring or equipping additional rolling stock, or changing its motive power, or furnishing electricity to a town for light, or abolishing grade crossings, or paying betterment assessments for widening or otherwise altering streets, or complying with any requirements lawfully imposed, or making permanent investments or improvements, or acquiring any additional real or personal property necessary or convenient for its corporate objects, or refunding its funded debt, or paying money borrowed or indebtedness incurred for any of the foregoing purposes, or other similarly necessary and lawful purposes, may, in accordance with sections twenty-eight and twenty-nine of this chapter, sections fifty to fifty-two, inclusive, of chapter one hundred and fifty-nine and sections forty-seven to fifty-five, inclusive, of chapter one hundred and sixty, increase its capital stock or issue bonds, secured by mortgage or otherwise, to such an amount, beyond the amounts fixed and limited by its agreement of association or its charter, or by any special law, as the department shall determine will realize the amount which has been properly expended or will be properly required, and as the department shall approve for such of the purposes aforesaid as are set out in its petition to the department.

The department, in authorizing the issue of any bonds under this section, may prescribe the minimum price at which such bonds shall be sold, and may modify such price from time to time, as it deems proper. Whenever it authorizes or has approved the issue or sale of bonds of a face value in excess of the amount determined by it to have been properly expended or to be properly required, it may, in its order of approval, or at any time thereafter, require the company issuing such bonds to establish a sinking fund, estimated to realize at the maturity of said bonds a sum equal to the difference between the amounts for which such bonds were authorized or approved, and the face value of the bonds so authorized or approve therefor, and may designate a domestic trust company as trustee and custodian of such fund, and may from time to time change such trustee. Any agreement relative to said sinking fund, made between the street railway company and the trust company selected as such trustee, shall not be valid until approved by the department.