How does grade crossing preemption work?

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How does grade crossing preemption work?

Postby railfan1988 » Thu May 23, 2013 9:31 pm

Is there anyone on here who can explain exactly how traffic signal preemption at a grade crossing works? As is commonly known, when traffic signals are located very near a grade crossing, they are preempted whenever a train is approaching, meaning they provide a special sequence which allows vehicles queued up on the tracks to clear the tracks, prior to the activation of the crossing warning devices. What I would like to know is the process by which the traffic signals are notified that a train is approaching. The traffic signal controller must somehow be able to detect the presence of an approaching train, and then provide the preemption sequence. This is a question to which I have trying to find the answer to for quite some time now, and hopefully, someone on here will be able to provide it, perhaps someone with a background in traffic engineering and/or railroad systems engineering.
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Re: How does grade crossing preemption work?

Postby Freddy » Thu May 23, 2013 10:16 pm

I had a pre-emp on 1 crossing and I'd need to show you on a set of plans/prints how it worked. Basically the best I can explain it was when a train got on my approach circuit it tripped a relay in the the traffic light control case which set the traffic lights red at the same time my xing activated. I hated pre-emps because if a xing ever got in trouble from bad weather or a battery
charger failure street traffic would get snarled up and by the time I got there the cops were primed and ready to jump in my ass even though my xing had no gates, just flashers.
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Re: How does grade crossing preemption work?

Postby swsrailguy » Tue May 28, 2013 12:23 pm

There are many different ways a grade crossing can work, so I will explain in pretty generic terms:

A highway crossing works by having some system (the different systems vary greatly) which can detect the presence of a train when it is approaching the road crossing. When the system determines the train is approaching, a crossing-relay drops out and the gates are activates. Ideally, the system is setup for this to occur 30 seconds before the train reaches the road. If preemption is required, then the system will also control a preempt-relay. When the preempt relay drops out, a line circuit going from the railroad to the highway traffic controller will open up, initiating the traffic light's preemption sequence. This is typically setup to occur 60 second before the train reaches the road (30 seconds before the gates come down). This gives time for the cars to clear out and the traffic lights to sequence to stop.

In order to get the 60 seconds of warning time, the system of train detection needs to be able to 'see' twice as far. So when it sees the train is 60 seconds away, it tells the traffic lights to turn red. And when it sees the train is 30 seconds away, it tells the gates to come down.

So the highway-traffic system doesn't do any train detection. There is just an input on the traffic-light controller that is driven by the railroad circuits which starts their preemption sequence. The railroad crossing circuits then need to have both a long start (for the traffic lights) and a normal start (for the crossing gates).
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Re: How does grade crossing preemption work?

Postby railfan1988 » Wed May 29, 2013 12:24 pm

swsrailguy wrote:There are many different ways a grade crossing can work, so I will explain in pretty generic terms:

A highway crossing works by having some system (the different systems vary greatly) which can detect the presence of a train when it is approaching the road crossing. When the system determines the train is approaching, a crossing-relay drops out and the gates are activates. Ideally, the system is setup for this to occur 30 seconds before the train reaches the road. If preemption is required, then the system will also control a preempt-relay. When the preempt relay drops out, a line circuit going from the railroad to the highway traffic controller will open up, initiating the traffic light's preemption sequence. This is typically setup to occur 60 second before the train reaches the road (30 seconds before the gates come down). This gives time for the cars to clear out and the traffic lights to sequence to stop.

In order to get the 60 seconds of warning time, the system of train detection needs to be able to 'see' twice as far. So when it sees the train is 60 seconds away, it tells the traffic lights to turn red. And when it sees the train is 30 seconds away, it tells the gates to come down.

So the highway-traffic system doesn't do any train detection. There is just an input on the traffic-light controller that is driven by the railroad circuits which starts their preemption sequence. The railroad crossing circuits then need to have both a long start (for the traffic lights) and a normal start (for the crossing gates).


First off, it is my understanding that at your standard grade crossing, an approaching train is detected by a computer that is located inside a cabinet which sits next to the crossing (the computer detects the presence of an approaching train when the train comes into an approach circuit, and it then activates the crossing warning devices). So after giving this more thought, it seems to me that at a preemptive crossing, the computer, before activating the crossing signals, would send a message to the traffic signal controller, telling it to provide the preemption sequence. Also, I believe that the preemption sequence is timed, based on the speed of the train. So the computer must also tell the traffic signal controller how long to provide the preemption for. Am I on target here? Also, can you elaborate on what a "crossing relay" and a "preempt relay" are? If you do, I'll probably have a better understanding of this. In order for me to understand this entirely, I would probably have to study traffic engineering and railroad systems engineering, as well as electronics.
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Re: How does grade crossing preemption work?

Postby Freddy » Wed May 29, 2013 1:28 pm

What would probably help you more than us trying to explain it would be for you to do a Google search of Railroad signaling and also Google Saftran, as they provide most of the crossing
detection systems now in use.
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Re: How does grade crossing preemption work?

Postby swsrailguy » Thu May 30, 2013 6:41 am

railfan1988 wrote:First off, it is my understanding that at your standard grade crossing, an approaching train is detected by a computer that is located inside a cabinet which sits next to the crossing (the computer detects the presence of an approaching train when the train comes into an approach circuit, and it then activates the crossing warning devices). So after giving this more thought, it seems to me that at a preemptive crossing, the computer, before activating the crossing signals, would send a message to the traffic signal controller, telling it to provide the preemption sequence. Also, I believe that the preemption sequence is timed, based on the speed of the train. So the computer must also tell the traffic signal controller how long to provide the preemption for. Am I on target here? Also, can you elaborate on what a "crossing relay" and a "preempt relay" are? If you do, I'll probably have a better understanding of this. In order for me to understand this entirely, I would probably have to study traffic engineering and railroad systems engineering, as well as electronics.


You have the general idea correct. The 'computer' controlling the crossing detects the train and tells both the traffic signals (first) and the crossing gates (some time later) to activate. A relay is basically an electrically controlled switch. Imagine a computer electronically flipping a light switch on and off. Then imagine that light switch isn't turning on a lamp, but the crossing gates & flashers. That 'light switch' is a relay. There would be one light switch(crossing relay) which controls the crossing, and another (preempt relay) which starts the highway preemption sequence (keep in mind that all the highway traffic lights are controlled by a completely separate computer system, and the only real interface between them is that one 'light switch' which tells the traffic controller that a train will be arriving in approximately X seconds).

That's basically preemption in a nutshell. Now the actual workings of the crossing 'computer' (I keep using quotes because it may or may not even be a computerized device) and it's methods for detecting the train can vary greatly, from very simple, to extremely sophisticated. But that's for another day. :) I hope this answered your question.
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Re: How does grade crossing preemption work?

Postby railfan1988 » Sun Jul 01, 2018 9:37 pm

swsrailguy wrote:
railfan1988 wrote:First off, it is my understanding that at your standard grade crossing, an approaching train is detected by a computer that is located inside a cabinet which sits next to the crossing (the computer detects the presence of an approaching train when the train comes into an approach circuit, and it then activates the crossing warning devices). So after giving this more thought, it seems to me that at a preemptive crossing, the computer, before activating the crossing signals, would send a message to the traffic signal controller, telling it to provide the preemption sequence. Also, I believe that the preemption sequence is timed, based on the speed of the train. So the computer must also tell the traffic signal controller how long to provide the preemption for. Am I on target here? Also, can you elaborate on what a "crossing relay" and a "preempt relay" are? If you do, I'll probably have a better understanding of this. In order for me to understand this entirely, I would probably have to study traffic engineering and railroad systems engineering, as well as electronics.


You have the general idea correct. The 'computer' controlling the crossing detects the train and tells both the traffic signals (first) and the crossing gates (some time later) to activate. A relay is basically an electrically controlled switch. Imagine a computer electronically flipping a light switch on and off. Then imagine that light switch isn't turning on a lamp, but the crossing gates & flashers. That 'light switch' is a relay. There would be one light switch(crossing relay) which controls the crossing, and another (preempt relay) which starts the highway preemption sequence (keep in mind that all the highway traffic lights are controlled by a completely separate computer system, and the only real interface between them is that one 'light switch' which tells the traffic controller that a train will be arriving in approximately X seconds).

That's basically preemption in a nutshell. Now the actual workings of the crossing 'computer' (I keep using quotes because it may or may not even be a computerized device) and it's methods for detecting the train can vary greatly, from very simple, to extremely sophisticated. But that's for another day. :) I hope this answered your question.
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Re: How does grade crossing preemption work?

Postby Engineer Spike » Tue Aug 28, 2018 11:10 pm

How does preemption work with motion sensing crossing protection? I get the part about giving the traffic light controller the signal befor the gates. When slowing while approaching a crossing, sometimes the gates will cycle. How could the preemption circuit hold the lights red, if the gates go back up, then come down. If this happened, then the traffic lights would not have time to stop and clear all traffic. Is there also therefore a method to keep the preemption circuit active? This would give the system a chance to determine whether the railroad movement was just slowing, or if it would stop.
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Re: How does grade crossing preemption work?

Postby lstone19 » Thu Sep 13, 2018 10:30 pm

Engineer Spike wrote:How does preemption work with motion sensing crossing protection? I get the part about giving the traffic light controller the signal befor the gates. When slowing while approaching a crossing, sometimes the gates will cycle. How could the preemption circuit hold the lights red, if the gates go back up, then come down. If this happened, then the traffic lights would not have time to stop and clear all traffic. Is there also therefore a method to keep the preemption circuit active? This would give the system a chance to determine whether the railroad movement was just slowing, or if it would stop.


At every crossing I've seen in my area, the traffic light preemption happens simultaneous to the gates and flashers activating. So no issue of the gates cycling (which does happen when an engineer comes into the commuter station 1/2 east too fast for the crossing). Once the traffic signal is preempted, it goes through its standard preemption cycle of all red except for the leg coming from the crossing for a time long enough to clear that leg and the crossing, then green for the parallel road (with left turn arrows skipped) which stays that way until the train is gone. As soon as the gates start up, the traffic light also goes red for the parallel road and resumes normal operation starting with left arrows for the cross street. If cycling causes the gates to go up before it's made it to green for the parallel road and a certain minimum time that way, the traffic light continues that way before resuming normal operation. Should the train moving again re-initiate the crossing protection, the traffic light also re-initiates the preemption cycle.
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ex-N&W Sandusky, Ohio
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