How does a crossing gate circuit work?

One option is to use an audio frequency overlay system.  These systems can be used to detect the presence of a train and the direction of travel.  These systems do not require the cutting of rails and using isolated joints.  Also, because they are an audio frequency overlay they will not interfere with the low frequency track circuits.

-Ryan

I talked to a BNSF signalman a while back and he said some of the grade crossing systems are speed-sensitive to traffic -- i.e., the time period for the train to arrive at the grade crossing is constant. There's an electrical signal sent down the rails and the rate of drop in electrical resistance determines the speed of the approaching train and the signals activate accordingly. Even with welded rail for several miles, this would work as the further out a train is, the higher the resistance in the rails to the point that the signal would not activate even if a train was there. An approaching train that stops will cause the signal to de-activate because the resistance stops changing. I'd love to be able to duplicate this in a model context.

Another thing I found interesting is that the old battery/relay systems were set up so that the relay was held in an energized state and the approaching train would shunt off the power to the relays causing them to switch to the normally closed position and turn block signals red or activate grade crossing signals. This can be done in a model context using appropriate resistors and insulated sections or optical sensors.

The the early 1960's I remember kids finding something metal and putting it on the track....causing the signals to come on. Worked pretty much like the Marx signal I had on my layout at the time! Not familiar with modern systems.....

Thanks for the replies.  The rate of drop in resistance makes sense to me.  I would have never thought of that.  I wonder how lightening thats not even close triggers the gates too. It does it often here.

In simple terms:

An electrical pulse is sent out on one rail to the outer edge of the crossing circuit, where a shunt crosses it over to the other rail and returns it to the instrument case (the "house" with the crossing signal equipment inside).  Equipment inside the instrument case measures the time interval it takes for the pulse to return.  If a train comes toward the crossing and passes the shunt, the leading wheels and axle now become the shunt.  As the train continues to approach the crossing, the distance the pulse has to travel to cross over to the other rail and return to the instrument case keeps getting shorter, and the interval between "send" and "return" keeps getting shorter.  The equipment calculates the arrival time of the train and activates the warning devices about 25 seconds before the train's arrival.

If the train stops after passing the outer boundary of the crossing circuit, but before occupying the crossing, then the interval stops becoming shorter and remains constant.  When the equipment senses this it presumes that the train has stopped and de-activates the warning devices.  If the train starts moving toward the crossing, then the equipment again predicts the arrival time at the crossing and provides activation of the warning devices.

Thanks for the info.  So can lightening shunt the track even when not close?

I guess it could shunt the track for a brief moment, but the thing that lightning does is send a whopping pulse of voltage through the rail or the pole line.  This likes to leave the rail or pole line at the nearest connection and fry everything in the instrument case.  That's why there is always a lightning arrestor between the track and the instrument case and between the pole line and the instrument case.  This interrupts the progress of the high voltage pulse before it reaches the equipment.  All the lightning arrestors I ever saw were self-sacrificing, one-time devices that had to be replaced after having functioned, leaving the instrument case without any connection to the track or to commercial power until repaired.