How A Track Circuit Really Works

The purpose of this note is not to explain track circuits, which is a big order, but merely to explain more precisely what goes on when a track circuit is shunted by a train. The traditional wording, as in Webb's <i>Railroad Construction</i>, says "Therefore, when there is any considerable loss of current from one rail to the other, the relay will not be sufficiently energized, the local circuit will be broken, and the signal will automatically fall to danger." True enough, but this does not give one the understanding necessary to handle all the problems that may arise. Understanding is not necessary, of course; tinkering around and following known prescriptions may also be enough, but understanding may make the job easier.

I will use a DC track circuit energized by a battery as an example, because it is simple and easy to understand, and illustrates all the necessary principles. The first track circuits used a couple of gravity cells in a well at one end of the block. The current went from one terminal to one rail, down the rail to the other end of the block, through the winding of the track relay, into the other rail, and back along it to the other terminal of the battery, completing the circuit. The resistance of the relay winding was a couple of ohms, about equal to the internal resistance of the jars of gravity. The resistance of the rails was (hopefully) small in comparison. Enough current, about half an ampere, flowed to pick the relay up.

When a train came along, at either end of the block, the path through wheels and axles had a much smaller resistance than the path through the track relay, so more current flowed from the battery. The voltage drop in the internal resistance of the battery caused the terminal voltage to drop, and so the voltage between the rails dropped. With less voltage across the track relay, the armature dropped and the track circuit was officially shunted. The track relay and the battery can be connected at any location along the track circuit, even at the same point, the train can enter from either end or be anywhere in the block, and the same thing always happens.

If you simply replaced the gravity cells with alkali cells, the track circuit would no longer work. The alkali cells had a low internal resistance, and easily supplied the extra current for the wheels without dropping the voltage between the rails, so the track relay would not drop out, no matter how much current was diverted. Of course, what was done was that a two-ohm resistor was placed in series with the battery to substitute for the missing internal resistance of the gravity cells. Now the voltage between the rails would drop nicely when a train came along. To check the track circuit, you simply put your voltmeter across the rails and watch what happens when a train comes along. Certainly the added resistor limits the current through the battery, but its value strongly affects the behavior of the track circuit; it is a much more important component than simply a current limiter.

An electrical engineer would say that what is wanted is a current source, and, in fact, that is what a track circuit calls for. If the resistance of whatever is connected to a current source decreases and makes it easier for current to flow, a current source obliges by lowering its voltage so the same current flows. Since this is now flowing through the axles rather than the track relay, we have the desired effect. However you put it, the main thing is that the track relay drops out because the voltage between the rails drops. A series resistor turns a voltage source (the battery) into something of a current source. The track circuit is most sensitive when the resistance of the source is equal to the resistance of the track relay winding.

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Composed by J. B. Calvert
Last revised 16 June 1999