Section 26. The department may investigate and determine the fair value for any purpose of all the property of any common carrier rendering a public service subject to the supervision of the department, actually used or useful for the convenience of the public, whenever it deems the ascertainment of such value necessary to carry into effect any provision of this chapter, and may at any time make a revaluation of such property. In making any valuation under this section, the department shall have access to and may use any books, documents or records in the possession of any department or board of the commonwealth or any political subdivision thereof.