I ran across this via another group I read. Pasting it here.
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At a highway grade crossing, we put AC voltage at specific frequencies onto the rail and put special tuned shunts (called termination shunts or “starts”) that allow AC to pass, (but not DC), at specified distances each way from the crossing. Maximum allowed track speed at the location dictates how far we set these out. This creates a special circuit (we call the approach) that will activate the crossing signal but won’t affect “block” signaling, which are basically the signals trains see. (it get’s really complicated) In dark territory (non-signaled), we just use hard wire shunts for our approach circuits.
Basically what happens, when a train passes the start, a device called a grade crossing predictor (GCP) sees the train shunt the circuit. As the train approaches the crossing, the impedance in the track increases, the voltage decreases and the rate at which this happens is used to determine the speed of the train and at what point the crossing signals should be activated. If a train stops short of the island circuit (the small circuit at the crossing and a few feet immediately on either side of the crossing) the GCP will detect that the voltage has stabilized indicating motion has stopped. The GCP will then “time out” and raise the gates. Once the train starts to move forward again, the GCP will see the voltage start to decrease again and will once again activate the signal.
When a train occupies the island circuit, the voltage is at or very near zero. As long as a train occupies this section of track, whether it is moving or stopped, the signal will remain activated. Once a train has passed the crossing and exited the island, the GCP will cancel the signal because it sees the voltage start to climb as the impedance starts to drop because the train is leaving the circuit.
[other users talking about shorting between both rails]
The GCP will detect any shunt (or short) across the rails. Let’s say my track speed limit at this make believe location is 60 MPH. Now let’s say that my approaches go out 3000 feet in each direction from the edge of the road at the crossing. My GCP sees out to that 3000 feet and as far as it’s concerned, it is seeing out to 100% of it’s approach. Let’s assume that every 300 feet is 10% of the approach.
If I shunt the rails out at say 90% of the approach, the GCP is going to say “hey, there’s something out on my approach. I’d better start analyzing it.” It will see numbers that are stable and high, so it will most likely not activate the crossing.
If I drop a shunt at say, 45% of my approach, my GCP is going to say “oh shit!!! I’ve got something half way through my approach. How in the world did that slip by me? I’ve got some fairly low numbers here. Maybe I’d better activate the crossing, at least until I can better analyze the numbers” The crossing may activate for a short time and then “time out” when it realizes that the numbers are stabilized. When it “times out” the warning device will be canceled.
If I drop a shunt inside the island (or just a few feet on either side of it) the crossing will activate and remain activated as an island shunt requires full crossing activation. Hope that clears it up for you!
We actually have special shunts that we use to purposely shunt the tracks for testing and different things. We can set them for “hardwire” mode (basically just a wire that clamps onto both rails) and we can set them up for “train shunt” mode, where they have .06 ohm resistance to emulate a train axle.
ETA, back in the day when we used DC or “ring 10” relay based approach circuits, your “jumper cables” would have activated the crossing anywhere in the approach.
