The differences between the two systems (ATS & ATC) ?


George, You might add to these, any number of other acronyms, such as ATP, ATO, ACS, ACES, ACSES, ITCS, etc, all currently in revenue service in North America, and all with lots of overlapping functionality. So it's hard to answer, given that nowadays different railroads (and agencies) throw these terms around in many different ways, often referring to different systems with the same acronym. For instance, BART uses the terms ATC, ATO, and ATP, to describe various parts of their fully automated system; while others use these same terms referring to very different systems.

And then comes along more alphabet soup with ARES, NAJPTC, CAS, CBTM, ETMS, V-ETMS, and finally PTC and PTS; each a form of "Communications Based Train Control" (CBTC). And so the beat goes on! Already, what Salt Lake city's Frontrunner is using, once called continuous inductive Automatic Train control (ATC); they appear to be calling ACS; while at the same time the Feds (FRA) have certified as satisfying the new federal requirement for PTS (the first in the nation). Yet it bears little resemblance to the PTS being installed by the UP, BNSF, or even Metrolink! In any event, what once seemed simple and straight-forward, can now be easily misunderstood.

So the best I can do is from a very general historical perspective, that suggests that "Automatic Train Stop" (ATS) generally referred to systems that either allowed a train to pass unencumbered; or attempted to automatically stop the train, or at least set off bells and whistles when the train passed a restrictive signal. It was "Go" or No Go.

Early on, this was accomplished by mechanical "trip stops", which would contact an onboard air valve lever if a train passed a stop signal, dumping the air in full emergency, thus quickly stopping the train. Such places as the NYCTA, and the Key system ferry pier (KEY, IER & SN) used this system very early on. But the obvious problem with that arrangement was that you had to run a red before the brakes were applied, requiring that at least two reds be behind every train, before the first yellow behind. This wastes track capacity, but is still in use today on much of the NYCTA, and other places.

A few decades later, "Intermittent Inductive Automatic Train Stop", also called ATS, was developed and installed over thousands of miles in North America. This system, often referred to by western railfans as the ATSF type, consisted of an electrically active onboard coil, mounted on the right side of the locomotive in the direction of travel, near railhead height.

When this coil passes over a similar, but passive coil mounted on the ties at each wayside signal, the active onboard coil could sense whether or not the wayside magnet coil was shunted out or not. It was shunted out, depending on whether the signal was displaying a clear or restrictive aspect. Any restrictive aspect would set off bells and whistles inside the cab and if not promptly responded to by a brake application, or otherwise "forestalled"; it would automatically set the brakes at full service, by itself. The great improvement here was that failing to respond to a yellow aspect would in itself cause the train to be stopped before any red could be violated. Moreover, double reds were not required to protect the trip stop, improving track capacity.

This was a major improvement to safety, saving many thousands from accidental death. But it is and was not perfect; the major downside being that after the initial reaction or forestalling action was accomplished, the locomotive engineer could still easily do whatever he wanted, and consequently still could fail to stop in time. Moreover, the circuitry causing the wayside magnet coil to be shunted was inherently NOT failsafe.


What was commonly referred to as ATC, was actually "Continuous Inductive Automatic Train Control". Its most notable feature is reliance on coded electrical signals sent through through the rails, to be inductively picked up by the train, and often also by the next signal back. These coded electrical signals usually consisted of a timed pulse code, riding on pulsed DC or pulsed AC current, and more recently, audio frequency pulses. In most systems, each of the many aspects that could be sent corresponded to a specific number of pulses per time period. But BART's ATC was a bit different, in that the code consisted of a digital "6 bit comma free audio FSK code" sent three times per second. Eight aspects were possible.

Continuous cab signaling was now possible, eliminating the need for intermediate wayside signals (though many roads kept them). But most importantly, the speed approaching the next signal could now be directly indicated on board, and enforced on board to whatever degree was desired. Indeed, as BART proved, even fully automated train control was now possible; proper speed being attained and maintained without human adjustment of any throttle or motorman's controller. Note that the BART experience also proves that fully automated control produces no particular economic benefit, and is thus not worth the costs involved.

Sadly, most roads (but not all) tried to limit the application of "signal enforcement" to only passenger trains, using only the cab signal function for freight. I do not believe such a limited application of the technology should be referred to as ATC, but it often is. At least the UP seems to agree. They call theirs ACS. I suppose that stands for "Automatic Cab signal"). In my view, the ICC goofed back in 1951(53) by setting the non ATC/ATS speed limit at 79mph. That allowed the railroads to get away with not equipping freight trains (except for FEC, which now employs ATC anyway); and then just lowering passenger train speeds, which they did, destroying their future viability. They then for the most part, got away with ripping out most ATC/ATS. The limit should have been set at 39mph! Which due to basic newtonian physics, is near the break point between controllable verses uncontrolled train motion, anyways!


The most advanced North American ATC systems, that of the NEC NY to Boston, sometimes referred to as ACSES; and that of the Salt Lake City Frontrunner, refered to as ACS; both apply all possible speed and braking enforcement functions, and are fully capable of total prevention of red signal running. That is of course, as long as they are cut in and on line. They are capable of this because restrictive aspects cannot just be forestalled. They must be obeyed immediately (within 8 seconds). Then the brakes must be continuously applied at the specified rate until the train actually attains the newly commanded speed. If released in error, bells will ring and if not acted upon - full service braking is irrevokably applied, requiring a complete ATC reset after being stopped.

Harmon GE's ITCS (a CBTC system) goes even further, overlaying over the coded track circuits, a wayside to train radio communication system, keeping the train apprised of additional dynamic data, such as the status of more distant signals, grade crossings, various hazard detectors, slow orders, etc; thus permitting higher speeds (up to 110mph) than the original system on that track was designed for. The ITCS otherwise functions identical to the other ATC systems described above.

George - I hope all that blather helps - at least a little!

OPB