Author Topic: What triggers crossing lights/gates?  (Read 1971 times)

hossman617

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What triggers crossing lights/gates?
« on: June 26, 2013, 10:40:12 PM »
As I type this while sitting on my grandfathers front porch in Orleans I can hear the warning bells from one of the 3 crossings that have flashing lights. They have been on for at least 15 minutes now, and it's the Hoosier sub so obviously I can't expect there to actually be a train. This isn't the first time they have done this, so my question is what triggers crossing lights, and how are these particular lights being accidentally set off?

UPDATE: A worker from CSX just arrived, and it took him all of 30 seconds to shut them off, without even touching the lights themselves. This only furthers my curiosity.
« Last Edit: June 26, 2013, 11:01:48 PM by hossman617 »

SX1979

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Re: What triggers crossing lights/gates?
« Reply #1 on: June 27, 2013, 01:54:24 AM »
I'm not sure of the mechanics of it, but I remember about 25 years ago while I lived next to the CSX in Vincennes. The neighbor that lived across the tracks from me gave me an old set of box springs for one of my kid's beds, and when I walked across the tracks I sat the box springs across the rails to balance myself. When I did this, the crossing gates went down and the warning  lights started flashing at the crossing about 75 yards away. I stood there for a good 20 minutes making the gates go up and down until I finally got bored and went in the house :)

doublestacks

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Re: What triggers crossing lights/gates?
« Reply #2 on: June 27, 2013, 04:24:52 AM »
LOL @ SX
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rrnut282

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Re: What triggers crossing lights/gates?
« Reply #3 on: June 27, 2013, 08:03:23 AM »
There are two types of activation, but both rely on the wheelsets of the locomotive or cars completing an electrical circuit ffrom one rail to the other.

In your case, I would guess a stuck relay in the cabinet was the culprit.
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mlb

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Re: What triggers crossing lights/gates?
« Reply #4 on: June 27, 2013, 09:11:31 AM »
When I Worked for the Erie, we would test all the gates and signals every Fri and fill out ICC logs. We had a metal spring loaded strip of metal that connected to the rails, this would simulate the train crossing the circuit and activating the lights and signals.

SemperVaporo

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Re: What triggers crossing lights/gates?
« Reply #5 on: June 27, 2013, 10:55:02 AM »
The lights, bells and gates are activated by a short circuit between the rails of the track.  The short being the wheels of the train (or someone laying a bed spring across the rails).
 
But, in reality, it is a bit more complicated than that.  It is not just a relay that is closed when current flows between the rails.
 
There is usually a short across the rails some distance from the crossing anyway, so current is always flowing, but because of the distance and the resistance of the metal, the current flow is low.
 
As a train crosses over that short and gets closer to the crossing, the current flow goes up as the short circuit caused by the train makes the distance shorter as the train moves.  The control circuitry in the small house/box next to the crossing is measuring the current, but not just how much is flowing, it is also measuring how fast it is changing.
 
The change in current flow, combined with the total current flow, is what triggers the crossing lights (etc.).  This way, if a train is traveling slow, the trigger doesn't occur until the current flow is high (based on total current flow) and if the train is traveling fast, the trigger occurs sooner (based on rate of change)... both ways the time between the trigger and the train arriving at the crossing is about the same.  I think the FRA mandates at least 20 seconds, and most RR's set it to closer to 30.
 
I think the voltage (causing the current flow) is around 24 or 48 volts, not enough to shock you or an animal that happens to touch both rails at the same time... and the current flow is limited such that a light bulb would not illuminate if you connected it across the rails.
 
Because the voltage is so relatively low and the current is so limited, and the circuit is so sensitive, any change in moisture will sometimes affect the trigger.  Rain, relative humidity, temperature (or even an animal relieving itself near the rails) can change the current flow just enough to make the circuit trigger the lights (etc.)
 
There are adjustments built-into the circuitry to vary the trigger point, both for total current and rate of change.  The signal maintainer probably just opened the door to the control box and barely jiggled one of the knobs to get the system to reset.  He does NOT want to go making large changes to the settings as that might shorten the trigger time to below the FRA mandate.
 
If major work is done to a crossing (or near it) the signal maintainer will have to get in his rail truck and drive through the crossing (with the rail to rail short on the truck enabled) at a fixed speed to test that the circuit trigger is set to the proper time.
 
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CSX2605

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Re: What triggers crossing lights/gates?
« Reply #6 on: June 27, 2013, 12:43:50 PM »
Also if the batteries go dead, the gate will drop, with no lights on, protecting the crossing even though there is a problem.


CSX2605

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Re: What triggers crossing lights/gates?
« Reply #7 on: June 27, 2013, 12:49:23 PM »
I'm not sure of the mechanics of it, but I remember about 25 years ago while I lived next to the CSX in Vincennes. The neighbor that lived across the tracks from me gave me an old set of box springs for one of my kid's beds, and when I walked across the tracks I sat the box springs across the rails to balance myself. When I did this, the crossing gates went down and the warning  lights started flashing at the crossing about 75 yards away. I stood there for a good 20 minutes making the gates go up and down until I finally got bored and went in the house :)
One evening, years ago, we were walking back to the farm and found a nice long rod of copper that evidently had been caught on a boxcar when the local worked Essex. We picked it up to take it home for scrap and while doing so the rod shorted the rail causing the gates to activate. Scared us to death but soon realized what we had done and continued on with our prize. Only bad thing was we probably threw on a track light for the dispatcher and hopefully not in front of a train.  :o

indyspy

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Re: What triggers crossing lights/gates?
« Reply #8 on: June 27, 2013, 03:12:24 PM »
The lights, bells and gates are activated by a short circuit between the rails of the track.  The short being the wheels of the train (or someone laying a bed spring across the rails).
 
But, in reality, it is a bit more complicated than that.  It is not just a relay that is closed when current flows between the rails.
 
There is usually a short across the rails some distance from the crossing anyway, so current is always flowing, but because of the distance and the resistance of the metal, the current flow is low.
 
As a train crosses over that short and gets closer to the crossing, the current flow goes up as the short circuit caused by the train makes the distance shorter as the train moves.  The control circuitry in the small house/box next to the crossing is measuring the current, but not just how much is flowing, it is also measuring how fast it is changing.
 
The change in current flow, combined with the total current flow, is what triggers the crossing lights (etc.).  This way, if a train is traveling slow, the trigger doesn't occur until the current flow is high (based on total current flow) and if the train is traveling fast, the trigger occurs sooner (based on rate of change)... both ways the time between the trigger and the train arriving at the crossing is about the same.  I think the FRA mandates at least 20 seconds, and most RR's set it to closer to 30.
 
I think the voltage (causing the current flow) is around 24 or 48 volts, not enough to shock you or an animal that happens to touch both rails at the same time... and the current flow is limited such that a light bulb would not illuminate if you connected it across the rails.
 
Because the voltage is so relatively low and the current is so limited, and the circuit is so sensitive, any change in moisture will sometimes affect the trigger.  Rain, relative humidity, temperature (or even an animal relieving itself near the rails) can change the current flow just enough to make the circuit trigger the lights (etc.)
 

That is a modern predictor circuit. And your description is correct for those. HOWEVER. I don't think the monon had a lot of those. (maybe only state road crossings)

I suspect the monon has a lot of the old "island" type. (The LIRC and INRD have a ton of these) These type have three circuits. One on each side of the circuit and one right at the crossing itself.

Once the train shunts the first circuit the gates go down no matter how fast the train is going. The train shunts the second and then third circuits. However once the first two circuits are clear, the gates will go up even if the last circuit is still shunted.

There are a couple other types, one that is only for crossings inside the limits of a CTC control point. (Some CR CP drop the gates as soon as a signal is lined CP-AN is like this, CP-280 was like this for the belt track)

Also to save a few bucks, it was common to put multiple crossings on one island if they were close in. Lawrence IN used to be like this. All three sets of gates would go up and down in unison for all three streets.
« Last Edit: June 27, 2013, 03:31:37 PM by indyspy »
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Hoot

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Re: What triggers crossing lights/gates?
« Reply #9 on: June 27, 2013, 04:54:48 PM »
Well, speaking from a maintainers view, there are several ways to activate a crossing. Predictor, motion sensor, afo circuits, dc tracks circuits, ring 10 track circuits, all low voltage ac or dc (3-8 volts). Crossings act diffently because they are setup different. Trouble with crossings can be alot, or simple. Hard to tell what is going on at crossing unless you study the prints for that crossing. Some crossing time out, some do not. Some have longer warning time, some programed for 30 seconds. Just becuase you might understand one crossing, doesnt not mean you understand the next one. Theres a reason class ones now have a one year apprentice program for maintainers, but still takes another five or so to be good. You want to know something, pm me, and i will try to answer your question.
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IndyKing

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Re: What triggers crossing lights/gates?
« Reply #10 on: June 27, 2013, 06:31:06 PM »
The change in current flow, combined with the total current flow, is what triggers the crossing lights (etc.).  This way, if a train is traveling slow, the trigger doesn't occur until the current flow is high (based on total current flow) and if the train is traveling fast, the trigger occurs sooner (based on rate of change)... both ways the time between the trigger and the train arriving at the crossing is about the same.

I never knew that, although it makes perfect sense.  I have seen trains stop close (~150 yards) to a local street crossing in order pick up the conductor and in doing so trigger the flashers, etc., only for them to deactivate when the train has come to a complete stop.  Once the train starts moving again, the flashers, etc. almost immediately re-activate as the train is so close to the crossing, albeit moving slowly.
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trainmaster53

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Re: What triggers crossing lights/gates?
« Reply #11 on: June 27, 2013, 10:10:43 PM »
Was told years ago by a Vice President of the Santa Fe Railroad what to do in an Emergency. I Have done that a Couple of times in the Fire Service. One way is to Use a Flare and move it in a Left to Right Movement across in front of you. The Other way was to Use a Jumper Cable and Attach the Cable to Each Side of the Rail. This will Drop the Signals in the Area and alert the Dispatcher to a Problem. Had that been done down in Texas, It would have prevented a Train from Striking a Low Boy that was Stuck on the Railroad in a Blind Curve. Had that Happen on Warman Ave in the Conrail Days. The Yard Master from the Transfer Yard came over and Wanted to Know who done it. I was the Guilty Party and he wanted to know who told me how to do that. When I told him, he Understood the Reason. The Low Boy was taking a Crane to the Link Belt Plant and got Stuck on the Tracks. Had the Indianapolis Police Department and Conrail Track Inspectors, Train Masters, Railroad Police and all kinds of Folks at Crossing at Warman Ave. After Everything was said and done Everyone was happy that what I had Done  and Prevented a Major Accident. Another Time I stopped a Freight Train Using a Flare on Lynhurst Drive just South of US 36 at a Car Accident with Power Lines Down. I was on the Wayne Twp Engine that Responded. After the Train Stopped and the Conductor came back to see why we stopped him. He just about Flipped. IPL Came and Raised the Power Lines up so the Train could Pass Safely. The Railroad was down for about 3 Hours, While IPL Replaced a Pole.

CSX2605

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Re: What triggers crossing lights/gates?
« Reply #12 on: June 27, 2013, 10:23:50 PM »
Lightning also triggers some crossing gates to go down!  :o

Dustin

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Re: What triggers crossing lights/gates?
« Reply #13 on: June 28, 2013, 07:54:49 AM »
Lightning also triggers some crossing gates to go down!  :o

As do juveniles with a pair of jumper cables...

hallbf

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Re: What triggers crossing lights/gates?
« Reply #14 on: June 28, 2013, 10:37:38 AM »
I suspect the monon has a lot of the old "island" type. (The LIRC and INRD have a ton of these) These type have three circuits. One on each side of the circuit and one right at the crossing itself.

INCORRECT AND WRONG!!!!!

First off, an island circuit is the crossing track circuit that contains the roadway and in newer installations is typically 50' past the edge of pavement on either side.  What you described above isn't an island circuit but rather a typical 3 track directional stick crossing circuit with one of the three tracks being an island circuit. 

As for INRD having "a ton of these".  I would hardly call 6 out of 142 crossings a ton. 

The parts of the former Monon I have seen have a mix of Motion Sensors and Predictors. 

Back in the day, having crossing circuits in the middle of signaled territory was a bit of a headache but in those days men were men, labor was cheap, and railroads had a lot more manpower.  Before Audio Frequency Overlays (AFO) came about you would need to split up your wayside signal block DC track circuit into smaller sections for however many crossings you had in that block.  Then you would need to interface the line wires running along the track into every crossing so the signal system would know if any one of those track circuits were down.  AFO circuits allowed you to create track circuits whose limits were not defined by insulated joints in the track.  Thus you now did not need to split your DC track circuit into multiple sections needed for crossings.  This eliminated the need to interface line wire into the crossing as well (to an extent, depending on how the AFO circuits were arranged).

Out of the technology for the AFO arose the Motion Sensor.  A Motion Sensor has two track circuits, an approach (both sides of the crossing) and an island circuit.  Insulated joints are not needed with this type of circuit.  The circuit is termined at each end of the approach with either a piece of wire, a wideband shunt (just a capacitor) or a narrow band shunt (capacitor and inductor).  A wideband shunt blocks DC and passes AC so it terminates the AC signal from the Motion Sensor and does nothing to the DC current.  A narrow band shunt is tuned to a specific frequency and allows that frequency to pass while blocking others and DC.  A narrow band shunt is what allows multiple motion sensor (and predictor) circuits to overlap each other.

The next progression was Constant Warning Time/Predicition which is basically an enhancement of the Motion Sensor.  Others have explained the basic theory of operation of Motion Sensors and Predictors.

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Re: What triggers crossing lights/gates?
« Reply #15 on: June 28, 2013, 10:45:14 AM »
There are a couple other types, one that is only for crossings inside the limits of a CTC control point. (Some CR CP drop the gates as soon as a signal is lined CP-AN is like this, CP-280 was like this for the belt track)

CP AN only does this off the Sanky Outbound when a signal is lined.  All other movements lined on all other tracks, don't cause the gates to drop.

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SemperVaporo

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Re: What triggers crossing lights/gates?
« Reply #16 on: June 28, 2013, 03:54:22 PM »
{Snip}
Out of the technology for the AFO arose the Motion Sensor.  A Motion Sensor has two track circuits, an approach (both sides of the crossing) and an island circuit.  Insulated joints are not needed with this type of circuit.  The circuit is termined at each end of the approach with either a piece of wire, a wideband shunt (just a capacitor) or a narrow band shunt (capacitor and inductor).  A wideband shunt blocks DC and passes AC so it terminates the AC signal from the Motion Sensor and does nothing to the DC current.  A narrow band shunt is tuned to a specific frequency and allows that frequency to pass while blocking others and DC.  A narrow band shunt is what allows multiple motion sensor (and predictor) circuits to overlap each other.
{Snip}


I think you will find you got the action regarding DC backwards... A Capacitor blocks DC completely and passes AC (the higher frequencies better than the lower ones).  An Inductor (coil) passes DC relatively unhindered, and will pass AC but the higher the frequency the less able it is to pass.
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Sharky1

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Re: What triggers crossing lights/gates?
« Reply #17 on: June 29, 2013, 09:57:51 PM »
It appears to me that the speed of the coming train can be predicted by the changes in resistance as the train comes into the signal area.

As each wheel set crosses onto signaled track the resistance across the track falls a bit lower resulting in a stair step shaped resistance profile. By measuring the rate that these drops occur a speed can be established and timing of the signal "predicted". If the drops stop occurring than the train is stopped and no signal is needed unless the train has already entered the crossing.

The addition of new wheel sets to the signal circuit could be sensed by measuring the changes in current or voltage across a capacitor. Each additional wheel set would cause a temporary increase in current across the resistors and drop in voltage. This may be too much detail as I'm sure that there are many ways that this can be measured.

Anyway the rate of change, however measured would give the direction and speed of the moving train and any change in speed.

How close am I?

SemperVaporo

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Re: What triggers crossing lights/gates?
« Reply #18 on: June 29, 2013, 10:05:43 PM »
Sharky1:  That is what I described above.  But it is not so much the additional wheels in parallel across the track as it is simply the leading wheels getting closer to the crossing reducing the resistance of the circuit and thus the current going up.
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hallbf

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Re: What triggers crossing lights/gates?
« Reply #19 on: July 02, 2013, 09:34:31 PM »
I think you will find you got the action regarding DC backwards... A Capacitor blocks DC completely and passes AC (the higher frequencies better than the lower ones).  An Inductor (coil) passes DC relatively unhindered, and will pass AC but the higher the frequency the less able it is to pass.

A wideband shunt (a capacitor) is connected between the two opposite rails of the track and forms the termination for the AC signal coming from the crossing circuit and doesn't affect the DC track circuit.  So yes, it does block DC completely like I had said.  I was wrong stating that a Narrow Band Shunt was a capacitor and inductor.  I meant to say capacitor and resistor.  I was thinking about Dummy Loads, which are just inductors we place in the circuit to simulate a length of track.  That got me thinking about inductors when I was talking about Narrow Band Shunts.  We also use inductors in series with a track battery or the electronic equivalent (including ElectroCode) to block the AC signals from the crossing equipment travelling through the track battery or ElectroCode box and shorting out the crossing signal.  General Electric calls these Track Battery Reactors and Chokes on the ElectroCode equipment.