Pub. Util. Code, § 315

The commission shall investigate the cause of all accidents occurring within this State upon the property of any public utility or directly or indirectly arising from or connected with its maintenance or operation, resulting in loss of life or injury to person or property and requiring, in the judgment of the commission, investigation by it, and may make such order or recommendation with respect thereto as in its judgment seems just and reasonable. Neither the order or recommendation of the commission nor any accident report filed with the commission shall be admitted as evidence in any action for damages based on or arising out of such loss of life, or injury to person or property. Every public utility shall file with the commission, under such rules as the commission prescribes, a report of each accident so occurring of such kinds or classes as the commission from time to time designates.