How Do Crossing Gates Work?

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How Do Crossing Gates Work?

Postby l008com » Thu Aug 21, 2014 3:54 am

This is a very basic question, but I'm looking for details. I'm very curious, what is the exact mechanism that makes the gates go down? Are there multiple systems used depending on the type of crossing? I, for example, know of a train station in Reading, MA where the train starts moving and the gates don't come down until the train is just a few feet away from the crossing. But the train is going extremely slow. But does this mean if the train didn't stop at the station, the gates would just start flashing as the train blasted through the station? I doubt that's how it works. How do the gets know when to go down? And they also stay down for the right amount of time (generally). I'm curious how the system works. I assume it can't work on shorting out a circuit with the trains wheels, as that's how the block signaling works, so wouldn't the two interfere with one another?

I've always wondered this, and never bothered to ask until now!
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Re: How Do Crossing Gates Work?

Postby ExCon90 » Thu Aug 21, 2014 3:09 pm

I'm not familiar with Reading specifically, but a common practice is to have the crossing circuit begin almost at the crossing; a train which has stopped at the station will move very slowly, as you observed, until the circuit Is occupied and the gates go down, and then accelerate. At Santa Ana, Calif., where Amtrak Surfliner and LA Metrolink trains stop throughout the day, I noticed that the gates went down when a train was ready to leave, but before the train moved. When I got on my train I asked the conductor about that; when the train is ready to leave, the engineer punches in a 3-digit code on a keypad in the cab, which sends a radio signal to actuate the gates. I learned later that a similar system is used on the Blue Line in Los Angeles when the station stop is made before the crossing; the operator punches in the code when ready to leave, and the gates go down. (In San Diego the (Orange?) Line to Santee, which is used by freights at certain times, has wayside signals which inform the operator or engineer whether the gates are down or not -- if not, a station stop is required -- but I don't know how that works either.) On your other question above, I have an idea about how they deal with trains not stopping at the station, but I'm waiting to see whether we hear from some people actually involved with it because I'd like to know more about it myself. (Incidentally, if someone does respond, I've been told that there's some problem with introducing that process, which involves speed detection, on electrified lines, but I forget what the reason was.)
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Re: How Do Crossing Gates Work?

Postby Freddy » Thu Aug 21, 2014 7:20 pm

There are 2 systems that I'm familiar with. Predictor and Motion. Predictors 'look' and see how fast the train or equipment is approaching the crossing and times the crossing to go into detection accordingly. Motion see's when he comes into the crossing detection limits and as long as he's moving, no matter how fast, the crossing stay's operating. That's as simple as I can explain it and have it understood by the layman.
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Re: How Do Crossing Gates Work?

Postby Engineer Spike » Fri Aug 22, 2014 10:50 pm

Some of the for mentioned systems will time out the gates if the train stops. They also put the gates down based on the train's speed. This way a slow train will not keep the gates down too long, such as when a heavy freight is climbing a hill. This is the reason why many rule books say that a movement can't increase speed approaching a crossing. It that case the gates will not be down long enough.

The radio code makes the signal activate so they can move right on the highball, and not tiptoe to the crossing, while waiting for it to activate.

Some older systems just had a circuit which was long enough for the signal to activate soon enough for maximum speed. A slow freight would keep them on forever. This invites disaster. Motorists could get used to waiting for a long time. Then they run the gate. To their surprise, a fast passenger hits the crossing much sooner. We know how this would end.
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Re: How Do Crossing Gates Work?

Postby l008com » Sun Aug 24, 2014 1:32 am

Engineer Spike wrote:Then they run the gate. To their surprise, a fast passenger hits the crossing much sooner. We know how this would end.

The completely undamaged car would have completely destroyed the train, right? :-D
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Re: How Do Crossing Gates Work?

Postby l008com » Sun Aug 24, 2014 1:34 am

So can we get a little more technical. These simple circuits and these speed detection based systems, how do they actually work? How do they sense the presence and speed of a train? The manual activation I totally understand. The others, I still don't get what is actually going on. I do understand the logic behind them, and the trouble figuring out when a train is going to get to a crossing if it's speed changes, because you don't know if the speed is going to slow down, speed up, stay the same. But I don't understand how the actual mechanisms work?
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Re: How Do Crossing Gates Work?

Postby nomis » Sun Aug 24, 2014 10:27 am

Quick aside: predictor systems do not work in electrified territory. The changes in the AC current of the crossing detection are not detected as the rails function as the return circuit for the centenary/third rail.
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Re: How Do Crossing Gates Work?

Postby Freddy » Sun Aug 24, 2014 10:48 am

Xings work off DC, not AC.
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Re: How Do Crossing Gates Work?

Postby ericbardes » Thu Aug 28, 2014 4:37 pm

I ran across this page from the Federal Highway Administration that articulates the different detection circuit types.

http://safety.fhwa.dot.gov/xings/com_ro ... sec04b.htm
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Re: How Do Crossing Gates Work?

Postby Myrtone » Wed Oct 22, 2014 12:24 am

I do realise from plenty of photographic evidence and footage I've seen over the years shows that most grade crossings in North America are either ungated or only have automatic half barriers. Here in Australia, we used to have swing gates at most level crossings as was the case in the British Isles, but as for the crossings still in use, the gates (which covered the full road width on train approach) were replaced by half barriers, and in the name of "progress." In the British Isles, and many mainland European countries, many crossings have full, skirted barriers, and a lot more railways over there are fenced.
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