Whenever track conditions are such that temporary speed restrictions are deemed necessary, every railroad corporation subject to the jurisdiction of the commissioner, shall, in addition to all other normal operating orders and practices, immediately install and continue to maintain during the entire speed restricted period “Slow Speed” warning boards or signs of suitable sizes so as to be readily visible at all times to the engineer of an approaching train and located at a sufficient distance from the point to be protected to insure proper reduction from maximum authorized speed to the restricted speed as well as a similar “Resume Speed” warning board or sign located at a point beyond the restricted speed section as to indicate where normal speed may be safely resumed.