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Hey, Kev - no problem. I was just trying to clarify some of the involved phenomena - those two web sites I gave links for pretty much describe it. - - - - - - - As for signals - I don't know how they do it today, or in other locations, but when I was in college (late '60s) we knew of a grade crossing near Ann Arbor Michigan where shorting one rail to the other near the crossing set off the signals. (OK - I was one of those "kids" I mentioned.) Indeed, with metal wheels and axels, trains "short" the two rails... complete a connection ( just like turning on a light switch completes a connection) for the signal-control and the signal goes off. We walked up and down the rails, and found the point beyond where the signal no longer went off when shorted - there was a point there where the rail continued on after a small gap formed by the two rail sections not quite touching. I'm sure there was a similar gap a similar distance away from the crossing in the other direction, leaving this section of rail several hundred yards either direction from the crossing which was connected by a wire to the signal control box (we located the wire) and the opposite rail also was connected by another wire to the box. Then they put a relatively low voltage difference on the two wires, and anything shorting the rails completed the circuit and current flowed (through a relay no doubt back in 1965 - perhaps an electronic circuit today) and the crossing signal activated. We didn't measure the voltage - we should have to make the experiment complete. I expect the presence of a train in a block is sensed in a similar way, so the dispatcher's displays and the block signals can react appropriately. Hey, now - it wasn't vandalism. We were young Electrical Engineering majors, it was a low-traffic area, and it was late at night. We were conducting "experiments" to see how it all worked. :-) - - - - - - - In the late '70s I worked for Dayton Power and Light Company - I know they have arrestors on the power lines. Have you ever noticed in a thunder storm when you are in your house and lightning strikes somewhere in your neighborhood and your house lights go off for a fraction of a second? That's the arrestor shorting the lightning pulse (and the power-company-supplied AC) to ground momentarily. After the lightning strike dissipates, the arrestor opens up (simple power-company-supplied AC isn't enough voltage to arc the gap) and you resume receiving AC to your house. - - - - - - - Page two, paragraph three of the NASA site I referred to has the 3x temperature of sun's surface item (the paragraph on thunder). - - - - - - - You guys keep posting the marvelous train-related info here! This is a fascinating board. I've been a radio/electronics/computer/photography nut for most of my 58 years, but only recently bitten by the railfan bug - I got it from my grandson. I kick myself for not paying attention to trains when I was young - steam was still in regular commercial service and I was oblivious! What a shame. Chuck Somerville
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