If the Secretary of Transportation does not begin a civil action under
section 20112 of this title to enjoin the violation of a railroad safety regulation prescribed or order issued by the Secretary not later than 15 days after the date the Secretary receives notice of the violation and a request from a State authority participating in investigative and surveillance activities under
section 20105 of this title that the action be brought, the authority may bring a civil action in a district court of the United States to enjoin the violation. This subsection does not apply if the Secretary makes an affirmative written finding that the violation did not occur or that the action is not necessary because of other enforcement action taken by the Secretary related to the violation.