Just curious, Russ, you don't happen to sell, maintain or install the GPS type systems, in question, do you? As an aside, the airlines can fly, with GPS, but the amount of ATC's handling them, from ground, approaches, hand-off's, etc., you are talking perhaps dozens of people, to handle one flight. A single train can travel across a railroad system, with ABS rules in effect, without input from anyone. (in theory, the DS would have to set-up the CP's, or his operator would) As already mentioned, the sky is a big, three dimensional place. You can stack planes, fly them side by side, etc. Lot's of room up there, for mistakes, without consequenses. A single track mainline is no place for system errors, for untried technology. Sure, some mine railroad in Canada has used it, but the US rail system is not some smalltime mining operation, and this sure in the heck ain't Canada. Those GPS systems don't detect broken rails. Signals can, and do. They also know if a rail car is on the mainline, and will warn an approaching train. We already run trains, without signals. It's called "dark-territory", and it is no place to be, when the Stuff hits the fan. Wayside signals/cab signals in conjuntion with LSL is all that is needed in todays rail system. Price keeps it from becoming a reality, though. The technology is tried and true, and it DOES work, but railroads justify the continuing carnage, by blaming us for all that goes wrong, without stepping up to the plate, and implementing the technology that we already know works. I, for one, would be happier to never see GPS on board my train. The carriers already wish they "ride along" inside every train, every trip, watching our every move. This kind of thing certainly does not help, with regards to that. Save the GPS for the airlines, or tanks on the battlefield. You shouldn't try to re-invent the wheel, so to speak. Just my .02 cents, though. Regards