Last night, I was headed up to a store for groceries. I took my scanner with me, so that if anything was happening nearby, I could stop to see it. The scanner was silent, so I didn’t expect any trains.
On my way, I passed through White, Ga, which has a pair of road crossings through town. As I was passing through, the first crossing (which only has flashers, no gates) was flashing, so I sped up and turned down a road to the second (which has both flashers and gates) which was activated. I sat and waited for about 30 seconds, but then the gates went back up and the lights stopped flashing. So, dissapointedly, I made my way across, only to be startled by the crossing activating again and seeing the first crossing still flashing. So I sat again, only to find that the crossing once again went back up. The crossings repeatedly did this for over 45 minutes without any rail traffic within 5 miles of either crossing. After all this I finally got the information from each placard on the crossings, and called CSX and informed them of the problem, then left for the store. After about 30 minutes I heard over the scanner that the crossings were being “repaired”.
My question is why would these crossings be doing this? The temp last night was 17, but I don’t think that would be cold enough to cause problems. Does anyone here know why?
The crossing electronics were detecting a “false shunt.”
Here’s how a simple D.C. circuit works. A battery puts an electrical signal onto the rails at a measured distance from the crossing which equals the desired warning time for the signal times the maximum authorized track speed for the line – for example, if 30 seconds warning time is required, and the maximum speed is 30 mph, the approach circuit distance is 30 x 30 x 1.467 (conversion factor). At the other end of the circuit at the grade crossing, the current crosses between the rails via a cable, and passes through a relay coil, energizing it and causing it to pick up and open contacts. To complete the circuit the current after passing through the relay enters the opposite rail and travels back to the battery. An approaching train enters the approach circuit, shunts the rails, and because it provides a lower resistance path for the current, the current passing through the relay decreases and the relay drops out, closing the contacts. That completes a second circuit which starts the grade-crossing bells, flashers, and gates.
Current always leaks between the rails through the ties, ballast, and subgrade. The signal maintainer adjusts a resistor in the circuit so the current leaking between the rails is not quite enough to activate the system. When the track gets wet – or the temperature changes – the resistance in the ballast and ties between the rail may decrease, allowing enough current to leak between the relays to drop the relay that starts the grade-crossing signals. The fix is to turn up the adjustable resistor – but when the weather dries up or the temperature rises, the maintainer will have to come back and turn down the resistor so that the train activates it.
False shunts are a common problem with dirty ballast. In cold weather on a wet track the dirt particles freeze together trapping even more water and further lowering track resistance.
Thi
Thanks for the info, I’d love to hear more. The first crossing does that every now and then. This is the first time I’ve ever seen the second do this though. A third crossing about a 1/2 mile away was not affected. They (CSX) have been conducting a maintenence blitz down here lately, and today a ballast train was parked near White. Every crossing along the line from Emerson to Wyvern (about 10 miles) had recieved new flashers if neccesary, completely new roadway, and in one case new bells. It was really cool watching crews lay down new tarmac, and watching as the Fire Department cooled the tarmac down with a fire truck’s hose. It was pretty hectic for the MOW crews, because an average of 3 trains came by every hour for about 8 to 9 hours!
I can’t diagnose from afar but the ongoing track maintenance activities and the malfunctioning signals are probably not coincidence. New ballast, tamping, and other disturbance of the track bed changes the current leakage. Tamping activities can also damage the shunts or knock off rail bonds, either one of which would start the signals. The tamper might just knick the shunt cable or a rail bond, not enough to start the signals while the MOW forces are there, but a few hours later after a train or two goes by, the rail bond falls off or the shunt cable finally breaks in two.
Adjacent crossing signals are often “DAXed” to each other. DAX stands for “Downstream Adjacent Crossing.” DAXed signals enable the approach circuits to overlap and when one signal detects a train in its approach, it sends a signal to the DAXed crossing signal for it to start at the appropriate time. So when one signal sees a false shunt a DAXed signal may too.
RWM
The maximum speed limit through there is 60 mph, so I was worried that someone would get obliterated. I’ve seen people beat trains there by less then 5 seconds, it’s kind of a weird spot for people to do that though, the White Court House/Police station is 100 yards away with a clear view of both crossings.