All cars standing on spur tracks on any public street shall be unloaded and/or loaded within 72 hours after being delivered on said tracks, Sundays and holidays excepted, except in case of unavoidable delay. The failure of any person, firm or corporation to unload and/or load said car within said time shall subject such person, firm or corporation to a fine of $10 which fine shall be paid into the City Treasury, and the permit of such person, firm or corporation to use such tracks shall be suspended until said fine is paid.