Western Railroad Discussion > Just What Does Activate a RR Crossing Signal System?

For many years now (more than I will admit too) I have been under the impression that when a train approaches a RR highway crossing that some device on the track or in close proximity to the track is activated by the passing of the train and that this in essence is the method which activates the lights and audio sound warning system at the crossing. After viewing a specific video where a camera is installed on the locomotive, I know am confused as to just what it is that actually does start the cycle that triggers the crossing alarms.

Let me explain. In viewing a specific video of Pentrex titled Cab Ride Vol 1 their are several cases in the video where the train has stopped short of the crossing and the signal system is not activated. In fact, at one place, Center Street in Highgrove, CA, the locomotive is but a short distance from the street and the warning system is not activated. As the Engineer sounds his first horn blast prior to moving, it appears that the warning system for the crossing becomes activated. I would appreciate it if someone would explain exactly what does activate the warning system for RR crossings. Thanks for your input.

Jim

Most crossings work on a 3 circuit system. Basically there are two approaches, east and west on either side of the road. Then in the road is section called the island. When a westbound train approaches and enters the east approach circuit, the train shunts the rails (shorts them out) and this triggers a relay in the signal control box. The relay then triggers the crossing signal to activate. The train has a set period of time to reach the island before the crossing deactivates. This time can be set by the railroad based on train speed and so forth but 20 to 30 seconds seems to be typical. If the train reaches the island the gates will stay active until the train leaves the island. The circuit also establishes the direction of the train and therefore will deactivate the crossing once the train leaves the east approach and island even though it is still in the west approach. This prevents the crossing from reactivating. Now that is the basic operation.

Some crossings today have more circuitry to determine train speed so that the gates can trigger later for slower trains and earlier for faster ones. There are also some that listen for the train horn before triggering but those are rarely used as far as I know.

A signal maintainer will have much more input on this but the above is the basic operation.

Rio Grande Fan
Denver, CO



Edited 1 time(s). Last edit at 09/16/11 11:58 by RioGrandeFan.

I did a Google search on "railroad crossing signals" and found this, which gets rather technical but it might answer your question:

http://matt.zont.org/signals/crossings/xngworks/xngworks.html

Scroll down about halfway to the Detection Circuits section.

Bill G.

Here's the activation device the Espee used on McAndrews Road in Medford, OR, before the overpass went in.

Fred Harrison
Central Point, OR
CORPpower/JSS/EORS



Edited 1 time(s). Last edit at 09/16/11 09:44 by fredkharrison.

---

Some signalmen would refer to the island circuit as the "hell or high water" circuit, as in, "come hell or high water, if there's a train on that track those gates better be down"!

Chooch Wrote:
-------------------------------------------------------
> For many years now (more than I will admit too) I
> have been under the impression that when a train
> approaches a RR highway crossing that some device
> on the track or in close proximity to the track is
> activated by the passing of the train and that
> this in essence is the method which activates the
> lights and audio sound warning system at the
> crossing. After viewing a specific video where a
> camera is installed on the locomotive, I know am
> confused as to just what it is that actually does
> start the cycle that triggers the crossing alarms.
>
>
> Let me explain. In viewing a specific video of
> Pentrex titled Cab Ride Vol 1 their are several
> cases in the video where the train has stopped
> short of the crossing and the signal system is not
> activated. In fact, at one place, Center Street in
> Highgrove, CA, the locomotive is but a short
> distance from the street and the warning system is
> not activated. As the Engineer sounds his first
> horn blast prior to moving, it appears that the
> warning system for the crossing becomes activated.
> I would appreciate it if someone would explain
> exactly what does activate the warning system for
> RR crossings. Thanks for your input.
>
> Jim

does the vid show the train already stopped or approaching the crossing with the crossing apparatus not activated ... if it's stopped the crossing protection timed out due to no train movement ... if it was approaching and the protection wasn't activated then that protection has definitely malfunctioned

If you're going slow enough, preparing to stop short of the crossing, then they might not go down until you are within 3-4 car lengths of it, if at all. After that, they will usually still time out if you don't occupy the street.

Complex circuitry to keep from annoying motorists.

In some of the newer systems, low voltage current passes down the rails. As the train approaches the crossing the computer identifies the train speed and activates the gates approximately 20 seconds before the train arrives at the crossing. The speed is judged by the time it takes for the voltage to go out and come back with the train shunting the track. A very simple explanation.

Mike Root
Madras, OR

NYC6001 Wrote:
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> If you're going slow enough, preparing to stop
> short of the crossing, then they might not go down
> until you are within 3-4 car lengths of it, if at
> all. After that, they will usually still time out
> if you don't occupy the street.
>
> Complex circuitry to keep from annoying motorists.

In at least two situations, NJ Transit uses a two step process for crossing gates.

At Ramsey on the Main Line, the track at the north end of the station is crossed by the town's busiest road. As a northbound train approaches, the gates go down. If the train speeds through, the gates stay down. If the train stops at Ramsey, the gates go up, and traffic crosses.

When the train is ready to head north, it pulls ahead about 20 feet, and stops. The bells and lights go on, and the gates go down. Then the train accelerates. I believe the signal logic is similar to the gates "timing out" when the train stops.

I posted a picture on the passenger thread about a car which ended up with the Ramsey crossing gate on top, and the P42 clearing it by inches.

Chooch Wrote:
-------------------------------------------------------
> For many years now (more than I will admit too) I
> have been under the impression that when a train
> approaches a RR highway crossing that some device
> on the track or in close proximity to the track is
> activated by the passing of the train and that
> this in essence is the method which activates the
> lights and audio sound warning system at the
> crossing. After viewing a specific video where a
> camera is installed on the locomotive, I know am
> confused as to just what it is that actually does
> start the cycle that triggers the crossing alarms.
>
>
> Let me explain. In viewing a specific video of
> Pentrex titled Cab Ride Vol 1 their are several
> cases in the video where the train has stopped
> short of the crossing and the signal system is not
> activated. In fact, at one place, Center Street in
> Highgrove, CA, the locomotive is but a short
> distance from the street and the warning system is
> not activated. As the Engineer sounds his first
> horn blast prior to moving, it appears that the
> warning system for the crossing becomes activated.
> I would appreciate it if someone would explain
> exactly what does activate the warning system for
> RR crossings. Thanks for your input.
>
> Jim

There are three basic types of crossing circuits.

1) The first is called a DC Stick Circuit or "Easer-Weaser." It involves three separate track circuits separated by insulated joints.

Let's say a westbound train enters the east approach track. This shunts the east track causing the east track relay to drop, which in turn de-energizes the crossing relay (XR). De-energizing the XR in turn activates the bell, de-energizes the crossing gate relay (XGR) causing the gates to drop and de-energizes the crossing repeater relay (XRP). The latter feeds energy to a flasher (EOR) relay, which cycles the lights by swapping polarity through the lighting circuit.

Now the train reaches the island, which is always a separate circuit in its own right regardless of what kind of crossing control system is used. Whenever a train is in the island, the XR drops and crossing must remain activated until the island clears.

OK, so in this case the island track relay drops. Also, what's called a stick relay has picked at this time, in this case the west stick relay. So even though the train has entered the west approach track and the west track relay is down, the stick relay circuits "wraps" around the track relay contacts. As far as the rest of the crossing circuit is concerned, there's no train any more and the crossing recovers once the island is clear.

We call them Easer-Weaser for East Stick/West Stick, which has the mnenomic of ESR/WSR.

2) The second is called a motion detector. This is essentially a complicated on-off switch with a power supply for the XR or a motion detector relay whose contacts feed energy to the XR.

Motion detectors do their work on the track via AC current and monitor what's called "phase angle." (That's the relationship between inductive reactance and resistance. For an AC crossing circuit the relationship is more a parallel resistor/inductor circuit. Thus the phase angle is measured using current vectors rather than resistance/impedance vectors as in series.) A perfect phase angle in a wholly inductive reactance circuit is 90 degrees. But we don't live in a perfect world, and railroad tracks certainly aren't perfect. Dirt, mud, rust, poor ballast, salt, even wet ties create lots of ways for current to go elsewhere. Most phase angles I see are in the 65- to 79-degree range.

Anyway, the motion detector looks for a change in the phase angle, usually an increase of 2 percent, or a drop in the impedance distance (RX) of 0.2 volts. Doesn't take much to get that. As a train enters an AC crossing approach, the phase angle increases and the RX drops. As soon as the MD senses that change, it removes power from the relay and the crossing activates. When the phase angle begins to increase, the crossing recovers. But it will never recover while a train is occupying the island. The island has its own high frequency AC circuit.

It doesn't matter how fast or slow a train is moving in a motion detector crossing circuit, it's on and sometimes on for a long time when it's a slow train. However. ...

3) . .. it does matter with predictors. A predictor is a smarter kind of motion detector. Rather than activate the crossing immediately, the predictor uses the decreasing RX value to determine the train's speed. It then performs a time-versus-distance calculation and activates the crossing at a pre-set time before the train reaches the crossing. It's the same time for every train regardless of speed because we program the approach length and warning time into the predictor's CPU.

One caveat to the last sentence: If a train speeds up or slows down in the crossing approach, the predictor gets a little baffled. Also, if a train stops in the crossing approach, the crossing recovers after pre-set time. The predictor then swtiches itself over to become a motion detector. As soon as it senses movement, a change in phase or RX, it activates immediately.

When the train clears the island, the predictor senses the rapid increase in RX and energizes the relay.

I won't go into pre-empts, interties, wraps, uni-directional, bi-directional, remotes, and auxililary crossing circuits. It's a lot of Inside Baseball.

shortlineboss Wrote:
-------------------------------------------------------
> In some of the newer systems, low voltage current
> passes down the rails.

It's always been that way for all crossing activation systems that rely on a train shunting a track

> As the train approaches
> the crossing the computer identifies the train
> speed and activates the gates approximately 20
> seconds before the train arrives at the crossing.

FRA minimum warning time is exactly 20 seconds. Most railroads shoot for 25 to 30 seconds.

> The speed is judged by the time it takes for the
> voltage to go out and come back with the train
> shunting the track.

Actually, Ohm's Law comes into play here. Predictors are constant-current devices. As the train gets closer to the crossing island the impedance decreases and with it occurs a proportional decrease in the AC voltage in the circuit. The rate of voltage decrease is what the predictor bases its time-versus-distance calculation on.

BTW, the voltage is always there, it does not "pulse" as I infer from your post.

For clarification purposes, the incident I describe in the Video I mention, the train is stopped at the crossing and I would estimate that the locomotive is no further than forty to fifty feet from the street berm. That's pretty close. Looking at the video and adjusting the speed of the video to slow, it appears that the sound of the two blasts of the locomotives horn (standard procedure for a train about to start) seems to activate the crossing lights and gates. The train is definitely stopped.

Jim

Chooch Wrote:
-------------------------------------------------------
> For clarification purposes, the incident I
> describe in the Video I mention, the train is
> stopped at the crossing and I would estimate that
> the locomotive is no further than forty to fifty
> feet from the street berm. That's pretty close.
> Looking at the video and adjusting the speed of
> the video to slow, it appears that the sound of
> the two blasts of the locomotives horn (standard
> procedure for a train about to start) seems to
> activate the crossing lights and gates. The train
> is definitely stopped.
>
> Jim

I'm not aware of an audio-activated crossing system. More than likely the locomotive moved just a few inches, perhaps nearly imperceptible from the camera's perspective, but that's enough to trigger the crossing.

signalmaintainer Wrote:
-------------------------------------------------------

> I'm not aware of an audio-activated crossing
> system. More than likely the locomotive moved just
> a few inches, perhaps nearly imperceptible from
> the camera's perspective, but that's enough to
> trigger the crossing.


The SP had horn activated gates. State Street Santa Barbara comes to mind.

ats90mph Wrote:
-------------------------------------------------------
> signalmaintainer Wrote:
> --------------------------------------------------
> -----
>
> > I'm not aware of an audio-activated crossing
> > system. More than likely the locomotive moved
> just
> > a few inches, perhaps nearly imperceptible from
> > the camera's perspective, but that's enough to
> > trigger the crossing.
>
>
> The SP had horn activated gates. State Street
> Santa Barbara comes to mind.

I recall that there were horn activated gates on the peninsula commuter line. They had microphones mounted on poles along the track that looked like speakers.

Clever, but I'm wondering how the system distinguished between a locomotive horn and a truck horn. Did it have to reach a certain decibel level in order to activate?

They were aimed right down the track so something off to the side, like a truck, probably wouldn't be "loud" enough to trigger the system.

PHall Wrote:
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> They were aimed right down the track so something
> off to the side, like a truck, probably wouldn't
> be "loud" enough to trigger the system.

Probably a unidirection microphone then. Make sense. I'd like to see something like that in person one day. Crossing warning systems have always fascinated me

signalmaintainer Wrote:
-------------------------------------------------------
> PHall Wrote:
> --------------------------------------------------
> -----
> > They were aimed right down the track so
> something
> > off to the side, like a truck, probably
> wouldn't
> > be "loud" enough to trigger the system.
>
> Probably a unidirection microphone then. Make
> sense. I'd like to see something like that in
> person one day. Crossing warning systems have
> always fascinated me

I can't find a photo of one, they kinda looked like a Federal Signal Thunderbolt siren:

http://www.flickr.com/photos/36329181@N05/3353831386/

(sorry the photo was too funny)

SP also had whistle controlled signals with the same "horn shaped" microphone.

ats90mph Wrote:
-------------------------------------------------------
>>
> I can't find a photo of one, they kinda looked
> like a Federal Signal Thunderbolt siren:
>
> http://www.flickr.com/photos/36329181@N05/33538313
> 86/
>
> (sorry the photo was too funny)
>
> SP also had whistle controlled signals with the
> same "horn shaped" microphone.

Yes, I recall that they were wedge shaped and even flatter than the Thunderbolt siren. They were mounted more or less at the height of the locomotive horn and were positioned near where the locomotive would usually stop.