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?
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 .
I thought it was the crossing gnomes.[8]
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.
I have a similar, very basic question: why is it that MOW high-railers
don’t set these signals (or intermediate block signals, for that
matter) off?
Regards, Joe
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?
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.
Yeah–I guess i didn’t phrase my question very well. I knew why railroads
don’t want MOW equipment to set off signals. I guess what I was really
asking is this–what is it about the design of MOW equipment that prevents
it from lighting these signals up? You’ve still got metal wheels on the rails,
so I don’t see why this wouldn’t make the signals go off (I’m admittedly
not much of an electronics expert).
Joe
(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.)
Thanks. I didn’t know any of that stuff.
Joe
Yeah, that might be a rather bad idea![8D]
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.
I had a crew ask to borrow my jumper cables when an electrically locked switch wouldn’t release so they could get out of the siding they were in. They wanted to “fool” the interlock into thinking there was a train on the main.
Been there done that. Ahh memories of my misspent youth.
Legal Note
I neither condone nor encourage anyone to connect jumper cables to railway tracks or engage in any other illegal act. The above post was a simple exercise in reminiscing and is not to be interpreted as any form of instruction.
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 dow