Shunting characteristics of the relay in an A-C. track circuit employed in railroad signaling

It is usually the practise in railroad signaling to automatically control the operation of signals and other safety devices in the system by means of a relay operated in an electrical circuit which is completed through the rails of the track. The track is sectionalized by placing insulated joints in the rails so that each section becomes insulated from adjoining sections. The rails are utilized throughout the entire length of a section as conductors for carrying current from a source of energy located at one end of the section to the track relay at the opposite end. The relay operates in a closed circuit and is normally energized. When a train enters a track section the wheels and axles of the cars form a conducting path for the current to flow directly from one rail to the other instead of flowing through the relay. This shunts the relay sufficiently to cause it to release its contacts thus allowing the signals to operate in such manner as to indicate the presence of the train. If the relay fails to shunt with a train in the section the signals not only fail to protect the train, but give false indications advising approaching trains that the track is clear. Continuous track circuits as they are employed in the present state of the art are recognized as the safest means of controlling signals, but even so where they are used the integrity of the entire signaling system is dependent upon the proper functioning of the track relay. It is therefore important to study the various characteristics of the track circuit particularly in connection with the shunting action of the relay.