>> To OPB: In another post on this subject, you mentioned systems that send encoded signals through the rails themselves.
>> Which technology is this, and would it have the ability to communicate " real time " information to the engineers in their locomotives ???
>> Or does this system only service signal & CTC functions ???

Coded track circuits have been around a very long time in one form or another, and at a minimum, provide a means to fail-safely communicate between adjacent signal locations without pole lines. Since the seventies and eighties, virtually all U.S. mainline trackage has taken advantage of this. With the addition of onboard equipment, cab signals can be provided at the very least. ATS is now just one step away.

At first, coding (or pulsing) was done by track relays. But Fully solid state and microprocessor driven versions started showing up in the early eighties, accelerating the wholesale elimination of pole lines nationwide. No more pole lines - no more burning poles! (sliding or falling down the pole, splintering and friction burning your face and body as you go down)

As to cab signals and ATS: Using coded circuits, the Pennsy installed it on much of their trackage in the east in the thirties. I seem to remember the NYC and NH having it as well. Further west, UP C&NW and CTA also used it on their busiest lines. The Bay Bridge Toll Bridge Authority installed it at the beginning in 1937. Onboard ATS was installed on the vehicles of all three tenants (key System, Interurban Electric, and Sacramento Northern). More recently, virtually all modern heavy rail rapid transit system have used it, starting with PATCO. Now Salt Lake City is using it on all their diesel locomotive hauled commuter lines. CalTrain is getting ready to implement it. That is by no means an exhaustive list. It it almost universal in Europe - The TGV for example, used the GRS version.

Brilliant as Dr Robinson's invention of 1878 was (closed loop track circuit), it had certain technical weaknesses. These were solved early in the last century by coding, or pulsing the energy applied to the rails at a slow rate, usually around 75 beats per minute. While that did solve the reliability problems, it was quickly realized that it also afforded the means to pass information along the rails to the next signal where it is decoded, by varying the pulse rate.

Since pulsing electrical current (AD or DC) passing through any conductor, such as rails, would also induce a pulsing magnetic field around itself (the rails); these pulses can be detected by a coil mounted on the locomotive. It is then a matter of decoding the pulse rate, and applying the data to control a cab signal (the same way you controlled the wayside signal). As at the wayside signal, the process for the cab signal would take only a couple of seconds, which of course, is carefully accounted for in the overall system design.

Add in an over-speed sensor capable cutting off the throttle and dumping the air, and you have ATS. While this is a simplified description, it is in essence just that simple - elegant really. Go even further, and add a speed servo onboard and high-def loops before absolute signals, and you have fully automatic train control - such as at BART and any number of other new rapid transits in North and South America.

While coded track circuits are now virtually universal throughout the United States, the difference between those used with cab signals and those used without (only wayside signals), is that if you are communicating only to the adjacent signals, you can program them to use whatever code interpretations you want; while if they must also support cab signals, they must use a standardized set of code rate interpretations for the whole district. This is generally a matter of programming them as they are installed, which is something you have to do anyway. But when I speak of reconfiguring for cab signals, that is generally the biggest part of retrofitting almost anywhere - changing the programmed pulse rates to be conform to a uniform plan. At some locations some additional hardware may be needed, but only off the shelf accessory devices - a need which is quite manageable.

To be sure, each railroad or authority is likely to choose differing code interpretations, or even use differing electrical methods entirely, presenting compatibility issues. But Amtrak has already been confronted with this problem and solved it with on-board equipment from Alstom (formerly GRS). It is capable of reading and interpreting well over a dozen different configurations. The only absolute requirement is that any given district must be uniform in its code interpretations.

George, I hope that answered your questions adequately.

OPB