Sand can still cause trouble with modern day signals. the relay coil is wired to the rails and is held in for a clear signal, when an axale bridges the two rails the current takes the path of least resistance and the relay looses power and the signal drops to red. sand can change the resistance enough to keep the relay energized in the green or clear position. i think most of the class one railroads equipment is new enough where this dosen't happen. the short line I worked on although modern and up to date had signal equipment from many different eras 1920's and up. the gates on one crossing we had would do this if you sanded thru it. the system works opposite what we model guys use when there is no power on the relay the signals go red and the crossings the gates go down (failsafe). hope this helps a little. Rick
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