Crossing arm/light activation

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storminorman

New Member
Because I work next to a double rail line I see many trains roll past in a days time. It's not unusual to see one stop off in the distance by the track side signal lights. Other times a unit may pull way forward much closer to the road crossing. I know about where it will trip the crossing arms/lights to come on, but how do they move past that location and park w/o tripping the arms/lights? Do they communicate with a control location that can manually control this?
 

Trackside

Plays Well With Others
I have a rough understanding of how it works, hopefully someone else can add more details:

At the crossing there is a bungalow (metal shed) which has all the computers/control for the crossing. An electrical signal is sent down one rail away from the crossing and when a train gets in range the metal axles of the train take that electrical signal and transmit it from one rail to the other which goes back to the computer at the bungalow. The speed and distance of the train can be calculated from the time it takes for the signal to return on the other rail.

Again others can add more detailed info, but from what I understand the system is set up so the gates are down for 20 seconds before the train arrives. That's why the gates go down so much sooner distance wise when the train is going fast compared to crawling along.

If the system realizes that the train stopped short than the gates go up, but I believe if the train is too close to the crossing than the gates stay down.
 

Crow

Member
It all depends on the type of detection equipment the crossing has. It sounds like this location has a constant warning device. It is able to determine the speed of the train, and is able to give about the same warning time for every train no matter what the speed. So if the train is going slow and then stops short of the crossing, the gate will never activate.

The great thing about a constant warning device is in an area where trains run at high speed such as 90 MPH, the train will receive the per-determined warning time, lets say 25 seconds. And a 10 MPH freight will also receive the same 25 second warning time. If they were using a motion sensor, or a 3 DC track circuit set up, the 90 MPH Freight would still get a 25 second warning time, but a 10 MPH Freight would get a 226 second warning time, which is 3 Minute, 46 second warning time.
 

Restricted Speed

EN09 ENGINEER IN TRAINING
I was told that the "shunt" in the rail is what causes the gates/lights to activate and speed is determined by how fast the axles are passing over the shunt in the rail. Not sure how true that is. I'll have to ask our signal maintainer if I ever see him....lol
 

Crow

Member
I was told that the "shunt" in the rail is what causes the gates/lights to activate and speed is determined by how fast the axles are passing over the shunt in the rail. Not sure how true that is. I'll have to ask our signal maintainer if I ever see him....lol
No, that isn't really how it operates. The constant warning time devices sends an AC frequency down the rail, and then shunt returns that specific frequency back down the the rail to the equipment. When a train crosses that shunt the wheels of the train shunt the rail and the equipment keeps the same current output on the rail, so it adjusts the voltage to keeps that same current. That change in voltage with other parameters programmed allows the equipment to determine the speed of the passing train, and then allows the equipment to provide the warning. If the train changes speed in the approach of the crossing, then the predicted warning time will change.
 

litz

Trainman
Note, also, that signals have a timeout, which allows them to release the lights/gates if a train is within the detection zone, but is not actually moving.

That's how you can nose right up to a crossing and stop, and the gates will time out and go back up. Soon as you move again, they go right back down.
 

Crow

Member
Note, also, that signals have a timeout, which allows them to release the lights/gates if a train is within the detection zone, but is not actually moving.

That's how you can nose right up to a crossing and stop, and the gates will time out and go back up. Soon as you move again, they go right back down.
Not all crossing have this feature. All newer crossing typically do. That was what was good about the invention of the motion sensor. You get the same warning time as a 3 DC track circuit, but if the motion stops, the crossing will time out and stop. But if it's a 3 DC track circuit, or a similar set up, once you step on the track circuit, the crossing will activate and stay until you either back off of the circuit, or cross the island and then pick up both the 1st track circuit and the island circuit. But in that case, if you never leave the 3rd track, and back across the same crossing you won't get any warning until you are back on the island because of the stick circuit.
 

Restricted Speed

EN09 ENGINEER IN TRAINING
Thanks for straightening me out guys.
Sorry I gave inaccurate info ;)

Some of the engineers I work with "THINK" they know everything...lol
 




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