hammerfang wrote:Interesting, how far down from the crossing do they usually place these trips? I assume a few hundred to a thousand feet or so, I'll have to be on the look out for them next time I'm waiting for a train.
The distance varies based on the speed of the line. The design is such that the lights are enabled and the gates drop with sufficient warning before a train enters the crossing. This would place the trip point half a mile away for a 30 second warning on a 60 MPH line. That point would have to be further away on faster lines for the same warning time or could be closer to the crossing on a slower line.
On signaled railroads the blocks used for triggering crossings are part of the signaling system. The shorting detection system noted earlier in this thread is used to detect something on the rails within each block. At the end of the block the rail is cut to isolate one block from the next (a short across the rails on one side of the cut is detected as a different block as the other side of the cut).
For a simple crossing with no crossings for miles you will find an "island" circuit that extends across the crossing for a short distance on either side (distance varies) and an approach circuit in each direction. Then additional circuits beyond the approach that don't affect the crossing in question. All of the circuits are tied together via relays to create a block for the signaling system (if one exists). If crossings are close together (for example, a quarter mile apart) the system gets complicated with overlapping approach and island blocks in different directions.
Probably your best chance of seeing the end of a circuit for yourself is at a crossing. Do some railfanning (staying OFF of railroad property). LOOK for where the train is when the lights and gates activate and use a watch or stopwatch to discover how much warning is being given. LOOK for where the end of the train is when the gates start going up again. The end of the island circuit will be between the point where the end of the train was and the crossing. If that point is visible from non-railroad property you should see the cut in the rail. (Don't trespass to see the cut. These cuts in the rail are also present near signals and interlockings. I've seen a few from station platforms where the public is expected and welcome. There are plenty of places to see infrastructure without trespass.)
A quick search found this
http://media.photobucket.com/image/rail ... G_2012.jpg . The gap will be filled in with non-conductive material.