silicon212 wrote: The direct DC block approach listed in post 2 is partially correct - however this section of track typically extends for about 20-50 yards on either side of the crossing.To understand how these work, it's a good idea to have a fundamental grasp on the Doppler effect, and what it does.Gated, lighted mainline crossings are equipped with some form of grade crossing computer - the computer will issue an AC signal over the DC blocks - the frequency sent back will vary from the signal produced and it is this variance that tells the computer whether the train is approaching or departing a crossing, and at what speed the movement is. The crossing computer, called a Grade Crossing Predictor, will use this information to determine train direction, location and speed, and will trigger the crossing warning within 30 seconds of arrival.It works like radar, except with a low frequency AC signal instead of an RF signal.
The direct DC block approach listed in post 2 is partially correct - however this section of track typically extends for about 20-50 yards on either side of the crossing.
To understand how these work, it's a good idea to have a fundamental grasp on the Doppler effect, and what it does.
Gated, lighted mainline crossings are equipped with some form of grade crossing computer - the computer will issue an AC signal over the DC blocks - the frequency sent back will vary from the signal produced and it is this variance that tells the computer whether the train is approaching or departing a crossing, and at what speed the movement is. The crossing computer, called a Grade Crossing Predictor, will use this information to determine train direction, location and speed, and will trigger the crossing warning within 30 seconds of arrival.
It works like radar, except with a low frequency AC signal instead of an RF signal.
Most newer crossings use this technology, however the older version did used fixed track sections, actually three circuits - an approach on each side and a circuit at the crossing. On these crossings, occupancy is absolute. If there is a shunt (car, train) in the circuit, the gates will be down. The length of the approach sections is based on track speed, making it possible for a train travelling faster than the design speed of the crossing to get to the crossing before the protection is in place.
There is a logic circuit involved. Once the train clears its approach circuit and the crossing circuit, the gates will lift, etc. If the train stops before it clears the opposite approach circuit, the crossing protection will again activate, and stay activated until the train does clear.
The Doppler-type circuit silicon212 describes will allow a train to approach a crossing, activating the protection, then stop short. A few moments after the train stops, the protection will shut down, raising the gates, etc.
For the speeders, etc, most rule books call for track cars to stop at all crossings.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
TrainManTy wrote: carknocker1 wrote: It isElectrical contact in the areas near the crossing , there is a small electric current in the rail . this area is isolated with insolators in the rail and when the wheels cross into this section of track the signals will go off . You can test this yourself by hooking jumper cables between the rails , but for several legal reasons I would not recommend it .Yeah, that might be a rather bad idea!
carknocker1 wrote: It isElectrical contact in the areas near the crossing , there is a small electric current in the rail . this area is isolated with insolators in the rail and when the wheels cross into this section of track the signals will go off . You can test this yourself by hooking jumper cables between the rails , but for several legal reasons I would not recommend it .
Yeah, that might be a rather bad idea!
Bucyrus wrote: silicon212,Thanks for that explanation. I always wondered how the signals knew how to clear the instant the train cleared, yet were set to activate when the train was a long way off. But I have another related question that I have raised before in other threads. Is there any possibility for the signals to fail to activate when a train is in the circuit? I know that the system is called, "fail safe," and has battery backup, but I am still wondering if it is absolutely 100% guaranteed to work. If it can fail to activate, how common or likely is it, and how would such a failure occur?
silicon212,
Thanks for that explanation. I always wondered how the signals knew how to clear the instant the train cleared, yet were set to activate when the train was a long way off. But I have another related question that I have raised before in other threads.
Is there any possibility for the signals to fail to activate when a train is in the circuit? I know that the system is called, "fail safe," and has battery backup, but I am still wondering if it is absolutely 100% guaranteed to work. If it can fail to activate, how common or likely is it, and how would such a failure occur?
Just from my experience crossing signals are not 100% I have seen lightning take them out, and also cause false activations. If we had a false activation or activation falure, and the dispatcher knew ahead of time, they would call us with the location of the crossing and we proceeded to the crossing and would be governed by the operationg rules.
mudchicken wrote:(1) The wheel contact patches on M/W equipment is much smaller. (Dime size vs. half dollar)(2) Most equipment is insulated at the bushings, the axle and contact plates to avoid putting stray energy into the track circuits. (Signal maintainers howl about adding to their track light calls fom the DS in CTC territory)(3) High rail gear on trucks is further insullated because the wheel surfaces have a 3/4 inch thick hard rubber tire on the standard hi-rail wheels, Steel flange contact is only intermittant. High rail gear sometimes have air activated shunts that drop wire brushes on the rail to get contact. (A major pain to maintain and keep operating correctly. Vehicle maintenance headache.)
(1) The wheel contact patches on M/W equipment is much smaller. (Dime size vs. half dollar)
(2) Most equipment is insulated at the bushings, the axle and contact plates to avoid putting stray energy into the track circuits. (Signal maintainers howl about adding to their track light calls fom the DS in CTC territory)
(3) High rail gear on trucks is further insullated because the wheel surfaces have a 3/4 inch thick hard rubber tire on the standard hi-rail wheels, Steel flange contact is only intermittant. High rail gear sometimes have air activated shunts that drop wire brushes on the rail to get contact. (A major pain to maintain and keep operating correctly. Vehicle maintenance headache.)
Dutchrailnut wrote: joemcspadden wrote:I have a similar, very basic question: why is it that MOW high-railersdon't set these signals (or intermediate block signals, for thatmatter) off?Regards, Joe They are designed so the signals and crossings dont get ativated. This is so work can be performed near crossings and also prevents them from showing up on signal systems, since they would not reliably shunt the tracks and could give Dispatchers false info.
joemcspadden wrote:I have a similar, very basic question: why is it that MOW high-railersdon't set these signals (or intermediate block signals, for thatmatter) off?Regards, Joe
They are designed so the signals and crossings dont get ativated. This is so work can be performed near crossings and also prevents them from showing up on signal systems, since they would not reliably shunt the tracks and could give Dispatchers false info.
B.Erdmann wrote: Hello, ive always wanted to get an answer to this question.well how do they know to go off? i thought maybe a censor, wieght on rails. but i dont think any are right. any one help?
Hello,
ive always wanted to get an answer to this question.
well how do they know to go off? i thought maybe a censor, wieght on rails.
but i dont think any are right.
any one help?
I thought it was the crossing gnomes.
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