Operators shall erect and maintain nonmechanical warning signs on both sides of the tracks at each intersection of the tracks and a street, sidewalk, alley or similar public crossing within the city limits, except where some mechanical sign, signal, device, or gate or flagman is required by resolution of the council. Such nonmechanical signs shall be of a height and size, and utilize such lettering, as to give adequate warning of such crossing. Whenever the council shall deem it necessary for the safety and convenience of the public that some mechanical sign, signal, device or gate should be erected and maintained, or flagman stationed at any street or other public crossing, the council, by resolution, shall order and direct the railroad company or companies concerned to erect and maintain such sign, signal, device, or gate or to station a flagman at such crossing at the expense of such company or companies. Any required flagman shall be stationed at such crossing during the periods of time of each day that the council shall designate. The resolution shall specify the street or other public crossing at which the sign, signal, device or gate shall be erected or flagman stationed. After the resolution has been adopted, a copy shall be served to the railroad company or companies with a notice of the time limit for compliance.