Track detection - snow, rain?

When a rail car sits on the track the detection circuitry see this 'short' and thus produces a track occupied condition. How come snow, and more so rain does not produce the electrical condition need to produce an occupied condition?

I am assuming if it was a simple case of current flow, then snow or rain can't produce a resistance/short very similiar to rail cars. Does the circuitry inject a certain frequency onto the rails, and thus the frequency sent back via a rail car 'loop' is different than the frequency sent back by water? Or maybe a rail car produces a better signal waveform, but water from rain would be grounded to earth and thus badly distorted or non-existent?

I've spent a fair bit of time reading about ABS, CTC, and other signaling systems, but havn't yet done much reading on track detection techniques.

Can anyone paste in some links to some web pages?

CROR410 wrote:When a rail car sits on the track the detection circuitry see this 'short' and thus produces a track occupied condition. How come snow, and more so rain does not produce the electrical condition need to produce an occupied condition?

I am assuming if it was a simple case of current flow, then snow or rain can't produce a resistance/short very similiar to rail cars.

Sounds like you pretty much answered your own question. For a track circuit to be closed there has to be a continuous flow of electricity (I think it's somewhere around 12 volts) between the two rails, and since snow and rain are poor electrical conductors in this instance they are not a factor.

Only problems they encounter is in winter when water and salt enter track circuits at RR Xings.

JLJ061 wrote: Sounds like you pretty much answered your own question. For a track circuit to be closed there has to be a continuous flow of electricity (I think it's somewhere around 12 volts) between the two rails, and since snow and rain are poor electrical conductors in this instance they are not a factor.

Yes it does sound like that, but that's not what I was saying. I was (trying) to say that (I think) the differences between the resistance of rolling stock versus rain/snow would be so minimal that detection could not be a simple case of current flow. Honestly I don't know.

I do know rain and snow are great conductors. My years of working with phone communications companies (including a CN telecom company) has demostrated countless numbr of times that circuits get shorted when made wet, with two fundamental different types of shorts: (1) conductor A is shorted to conductor B, and (2) either or both of the conductors is shorted to ground. In my mind these two scenarios also apply to track circuitry.

So thus my later assumptions that track detection circuitry must me more complex than a simple current loop test. I am actually thinking now that there is a series of differing tests that each produce their own state, and the logic of these tests is fed into a Boolean equation whose output produces the final state of the track.

rain and snow are poor conductors, they only conduct a few mili amps at 12 to 20 volt. a signal circuit uses fairly high current to pick up fairly big relays, nothing like the very low amerage the phone company works with.

The basic track circuit is a simple DC circuit based on the extremely low shunting resistance produced by a wheel and axle between the rails. This is why rusty rail is a problem.

The shunting effect of a train is on the order of 0.1 Ohms or less. The leakage resistance of even very wet ties and ballast is much greater than this other than the extreme case of very salty water in a road crossing.

Typical track circuit voltage is 2 volts, relay coil resistance is 4 Ohms and pickup current is on the order of 0.065 Amps. The track circuit loop resistance has to be kept low which is why all rail joints are bonded. A series resistance is added at the battery to limit maximum current when the train is directly shunting the battery feed point and another series resistance is used at the relay end of the circuit.

So, there no fancy logic in a basic DC track circuit, just a careful ballance of rail resistance, relay operating current and series resistances so that enough current flows through the relay to keep it energized when no train is on the circuit and the shunting action of the train diverts enough current to drop the relay.

More modern track circuits for crossing protection such as Motion Detectors and Predictors use high frequency AC signals and are a different situation with phase shift detection and logic circuits responding to train motion and also speed in the case of a Predictor.

DutchRailnut wrote:rain and snow are poor conductors, they only conduct a few mili amps at 12 to 20 volt. a signal circuit uses fairly high current to pick up fairly big relays, nothing like the very low amerage the phone company works with.

Amperage is a function of voltage / resistance. Since the resistance of water (rain, snow) is a constant this means the current flow for a shorted telco circuit which is 48VDC would be double than that of a shorted rail circuit at 24VDC. Regardess the amperage during normal circuit operations have little bearing when compared to a shorted circuit. There are way many more items to consider.

Regardless.....further research seems to indicate that rail circuity is indeed susceptable to wet weather and thus there is a requirement to engineer the length of rail, type of ballast (ie. 3K ohms for timbered sleepered track or 5K ohms for concrete sleepered track) with additional resistance circuitry (which can be calibrated). If the engineering is not right, or the calibration is off, false signals will be produced during wet track conditions. One common false signal is a signal not returning to clear once the track block is no longer occupied. There is also R&D happening to use fiber optic cables to detect broken rails, instead of using electrical measurements as well as other complex techniques.

So it seems the the answer is: Today, it is a complex case of measuring current flow and voltage levels.

CROR410 wrote: Regardless.....further research seems to indicate that rail circuity is indeed susceptable to wet weather and thus there is a requirement to engineer the length of rail, type of ballast (ie. 3K ohms for timbered sleepered track or 5K ohms for concrete sleepered track) with additional resistance circuitry (which can be calibrated). If the engineering is not right, or the calibration is off, false signals will be produced during wet track conditions. One common false signal is a signal not returning to clear once the track block is no longer occupied.

You are correct. The trick is to get the correct calibration to work under a wide range of "ballast resistance(leakage effects of wet ties and ballast)" conditions. There is a "hysteresis" effect (difference in pickup vs dropout currents) in a DC track relay. If ballast resistance drops to something on the order of 0.25 Ohms, sufficient current may be diverted so that the relay fails to pick up after a train has cleared the circuit.

Given the requirement that the circuit be "fail safe" and not falsely indicate that the track is intact and unoccupied, it is very difficult to find any AC or DC circuits that will not fail if an extreme leakage is present between the rails due to something such as salt water.

Clearblock, Does the modern electronics have 'auto-adjust' circuitry that can detect wet and dry periods, and adjust the some of the calibration circuitry automatically?

CROR410 wrote:

DutchRailnut wrote:rain and snow are poor conductors, they only conduct a few mili amps at 12 to 20 volt. a signal circuit uses fairly high current to pick up fairly big relays, nothing like the very low amerage the phone company works with.

Amperage is a function of voltage / resistance. Since the resistance of water (rain, snow) is a constant this means the current flow for a shorted telco circuit which is 48VDC would be double than that of a shorted rail circuit at 24VDC. Regardess the amperage during normal circuit operations have little bearing when compared to a shorted circuit. There are way many more items to consider.

Regardless.....further research seems to indicate that rail circuity is indeed susceptable to wet weather and thus there is a requirement to engineer the length of rail, type of ballast (ie. 3K ohms for timbered sleepered track or 5K ohms for concrete sleepered track) with additional resistance circuitry (which can be calibrated). If the engineering is not right, or the calibration is off, false signals will be produced during wet track conditions. One common false signal is a signal not returning to clear once the track block is no longer occupied. There is also R&D happening to use fiber optic cables to detect broken rails, instead of using electrical measurements as well as other complex techniques.

So it seems the the answer is: Today, it is a complex case of measuring current flow and voltage levels.

I suspect that the problems with shorts in the telco cicuits are between terminals or components in very close proximity versus the well over 4 feet between the rails. That much longer leakage path has a substantially higher resistance.

See if you can locate a copy of the book “The Elements of Railway Signaling” published by General Railway Signal. It contains a whole chapter devoted to this issue and goes into more detail than you will probably want to know!

However in simplest terms, reliable train detection depends on a careful balance of voltage applied across the rails, the pickup and dropout current characteristics of the track (detection) relay and the series current limiting resistance in the detection circuit.

But there are all sorts of confounding and often conflicting factors to contend with: rusty rails, ballast conductivity changes between wet and dry (also between heat and cold), conductive contaminants getting onto the ballast (dust, roadway deicing salt, etc). Also transient events such as lightning strikes, etc.

All things considered it’s amazing train detection works as reliably as it does…

CROR410 wrote:Clearblock, Does the modern electronics have 'auto-adjust' circuitry that can detect wet and dry periods, and adjust the some of the calibration circuitry automatically?

Not that I am aware of. The DC track circuits are still basicaly the same technology that has been used for years. The fail-safe requirement would seem to discourage any circuit that tried to automatically adjust for a perceived change in environmental conditions. The prefered method is to have the Signal Maintainer periodically inspect, calibrate and document any changes.

There are all kinds of modern technology that could be used to detect the presence of a train without the use of a track circuit. One reason for reluctance to use this type of technology is it eliminates the fail-safe detection of a broken rail inherent in any track circuit based system.

A quick answer to your comparison to telephone systems -

The telephone CO battery is 48 or 24 V and circuit loop resistances are on the order of thousands of Ohms. Consequently a leakage resistance of hundreds of Ohms would be significant compared to a track circuit where the leakage path would have to be a small fraction of an Ohm to affect the circuit.

Ken W2KB wrote:I suspect that the problems with shorts in the telco cicuits are between terminals or components in very close proximity versus the well over 4 feet between the rails. That much longer leakage path has a substantially higher resistance.

Nope. The shorts can be very close to the customer or very close to the Central Office, or anywhere in between, and a copper pair can very often be miles and miles of copper. We have actually measured something in the telco world known as a 'high resistance short'....sound like a contradary in terms, but it exists.

clearblock wrote:One reason for reluctance to use this type of technology is it eliminates the fail-safe detection of a broken rail inherent in any track circuit based system.

What's your thoughts on using a strand of fiber optic bonded to the rail for broken rail detection? The rail breaks, the fiber breaks. The fiber breaks, the light stops flowing into the receiver, thus alarm!

FarmallBob wrote:See if you can locate a copy of the book “The Elements of Railway Signaling” published by General Railway Signal. It contains a whole chapter devoted to this issue and goes into more detail than you will probably want to know!

Thanks!! I couldn't find it but I did find a book called Railroad Signaling so I just ordered it online!