A carrier who receives freight for shipment is bound to forward the freight within a reasonable time, even though his employees strike or otherwise refuse to work, provided that if the strike is accompanied with violence and intimidation so as to render it unsafe to forward the freight, the carrier shall be relieved as to liability for delay in delivering the freight, if the violence and armed resistance are of such character as could not be overcome by the carrier or controlled by the civil authorities when called upon by him.