It is unlawful for any person to operate or cause to be operated any train or locomotive in a backward direction along or over any public crossing within the city without causing some person to precede the train to keep the track clear and prevent accidents; except, that such person is not needed when the leading car of such train is equipped with an alarm whistle and an emergency brake valve in the proper working order which shall be operated by a trainman in the proper manner. (Prior code § 25-4)