The Invention of the Track Circuit / The history of Dr. William Robinson's invention of the track circuit, the fundamental unit which made possible our present automatic block signaling and interlocking systems

by American Railway Association

It is precisely in this way that the insulation of one rail from another is effected (Fig. 6) and the long, practical use of many hundred joints of this sort, has proved it to be a method both economical and thoroughly efficient.

Fig. 6

A much more secure joint, however, is obtained by insulating the existing iron fish-plates from the rails by means of heavy fiber plates, and their bolts from the rails by fiber bushings (Fig. 7). While this method is superior to the first mentioned one in that it makes a more secure rail joint, it is no more efficient as an insulation.

Fig. 7

One would naturally suppose that owing to the large surface of contact existing between the rails and their connecting or fish-plates, and from the apparent security of that contact obtained by the bolts through them, no trouble would be experienced by the current in passing from one rail to the other. This, however, is not the case, as the bolts and even the plates themselves frequently become loose, even when provided with the best of nut locks, and the rust and dirt settling between them and the rails oftentimes increase the resistance of a track section to a serious extent. Again, even when tightly bolted and locked, these plates form but an imperfect contact, owing to the scale or rust upon them. Therefore, to insure that the resistance of a track section may be as low and as constant as possible, we have found it absolutely necessary to connect each two adjacent rail ends together by means of a short piece of very strong wire (Fig. 8).

Fig. 8

These wires are termed "track wires" (bond wires) and are provided with a button-head rivet at each end, which is securely soldered thereto, for the purpose of securing them to the rails. (Bond wires are now attached to the rails by channel pins or are welded on.) The connections from the rails to the battery and relay of a track section are secured to the rails in the same manner. The battery is usually located in a chute or well sunk in the ground at the terminal of each section, which is provided with an elevator in which the battery is placed and by which it may be raised and lowered at will. All wires when placed underground are run in grooved lumber in order that they may be secure from injury.