For a simple track circuit its easy.

As been said, a shunt is a short circuit. A track circuit is designed so a relay will pick up at a certain voltage and drop out at a certain voltage. A battery is supplied at one end, one rail positive the other negative. At the other end the voltage goes to a relay.

(At each section or block are insulatated joints to keep said voltage from going past into another block. That is where the positive and negative rail switch sides so a bad joint can't pass a voltage that may pick up another relay) The signal department designs what the working voltage will be, hence the pick up and drop out voltage. They take into account the resistance,rail, leakage thru ballast, the block length, etc.

Now all of that is set up the relay that picks up or drops out has terminals on it. Those go to other equipment such as block signals, crossings, etc. Also taken in account if the track is a single direction or both directions. It is easy to add other functions to a block as desired.

For a crossing, (non-signaled territory) all it takes is 2 relays. A "stick" relay and a regular "XR" relay. When a train approaches from one direction it knocks down a relay. Say you are looking at the relay and the left side drops. When it does it also latches some center contacts. This drops the XR relay and the equipment starts working. As the train reaches the crossing there is a an island circuit. (one side of the road to the other)When it does it picks up and holds the center contacts keeping the equipment working. as the rear of the train exits the island circuit the island circuit drops out picking up the XR relay shutting off the equipment. the train continues until it exits the approach circuit on the other side. Then that set of contacts picks up, setting the stage for another train from either direction.

That is why in the old days if a train stopped clear of the crossing and then wanted to back up they had to flag traffic until the rear car re-entered the island circuit and started the equipment again.

Most automatic block signals only took 4 wires to operate in both directions. Maybe 2 more for battery charging or 110 volts to run chargers down the line. Most equipment was 10V DC and the relays were very low voltage. If you look at a track relay, it will say inside the glass when and where it was tested, and say 3.45 volt pick up and 2.95 volt drop out or something like that.

All the new stuff they are putting in is "Coded Track Circuits" and way beyond me as I understand but the final track circuit is still somewhat the same.

I am sure I have made a mistake or two but that's the way I understand it.


Nudge