swsrailguy wrote:There are many different ways a grade crossing can work, so I will explain in pretty generic terms:
A highway crossing works by having some system (the different systems vary greatly) which can detect the presence of a train when it is approaching the road crossing. When the system determines the train is approaching, a crossing-relay drops out and the gates are activates. Ideally, the system is setup for this to occur 30 seconds before the train reaches the road. If preemption is required, then the system will also control a preempt-relay. When the preempt relay drops out, a line circuit going from the railroad to the highway traffic controller will open up, initiating the traffic light's preemption sequence. This is typically setup to occur 60 second before the train reaches the road (30 seconds before the gates come down). This gives time for the cars to clear out and the traffic lights to sequence to stop.
In order to get the 60 seconds of warning time, the system of train detection needs to be able to 'see' twice as far. So when it sees the train is 60 seconds away, it tells the traffic lights to turn red. And when it sees the train is 30 seconds away, it tells the gates to come down.
So the highway-traffic system doesn't do any train detection. There is just an input on the traffic-light controller that is driven by the railroad circuits which starts their preemption sequence. The railroad crossing circuits then need to have both a long start (for the traffic lights) and a normal start (for the crossing gates).
First off, it is my understanding that at your standard grade crossing, an approaching train is detected by a computer that is located inside a cabinet which sits next to the crossing (the computer detects the presence of an approaching train when the train comes into an approach circuit, and it then activates the crossing warning devices). So after giving this more thought, it seems to me that at a preemptive crossing, the computer, before activating the crossing signals, would send a message to the traffic signal controller, telling it to provide the preemption sequence. Also, I believe that the preemption sequence is timed, based on the speed of the train. So the computer must also tell the traffic signal controller how long to provide the preemption for. Am I on target here? Also, can you elaborate on what a "crossing relay" and a "preempt relay" are? If you do, I'll probably have a better understanding of this. In order for me to understand this entirely, I would probably have to study traffic engineering and railroad systems engineering, as well as electronics.