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I'm no signal maintainer, but from what I vaguely remember from classroom discussions with the signals guy who was also our rules instructor, this applies to electrically insulated blocks. I may only have this partially right, so bear with me: A DC current runs in both rails using positive leads and is grounded through the roadbed and a grounding cable, as your friend wondered. The circuit is based on a phase detection loop. When the train completes the shunt, the voltage drop triggers the relays, which switch the circuit to AC. This also provides the "flashing" lights. Don't know the specific voltage or amps, but would imagine the amperage is fairly low. The signal doesn't "read" the location of the train, it simply reads the block occupancy. On Conrail, whenever ther was a problem with the shunt, they called this an ""intermittent TOL" or "Track Occupancy Light" which would show up on their screens as a flashing track block, and this would alert them to areas where they needed to send police or signals maintenance. ATS relay track occupancy to signals in front and behind the train (depending on whether it is 251 or 261 territory). On in-cab signals, an induction coil is mounted ahead of the wheel of the locomotive, and this picks up modulation through the track on electrical currents which are piggy-backed at certain frequencies. The induction is detected from the field created between the wheels and the track. PTC uses satellite tracking and GPS coordinates to constantly monitor actual train location and distance between trains, allowing for shorter blocks and/or more "flexible" block signalling. Signal power is provided by electrical utility with failsafe battery backup. Without going into the actual mechanics of the thing (so as not to provide others who may read this with sabotage information), the worst that is theoretically possible with signal failure is Restricted speed. Hope this helps. Joe
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