PTC vs ATS

Yes, ATC is out of date, plus it did/does not have GPS capability.

Maybe it will help if we clear up some things about ATS:

ATS is operated by wayside shoes, one at each block signal (and sometimes one at the location of the permanent slow board in advance of places where a large reduction in speed is required).  These are actually electromagnets which are energized if the adjacent signal displays a Proceed (green aspect) indication.  Another electromagnetic shoe mounted on the truck of the locomotive passes over the inductor with a very small clearance, and senses that the shoe is energized, and then does nothing.  If the signal is other than green, then the wayside shoe is no longer energized, and when the locomotive shoe passes above it, it detects a shoe (because it's steel) and, because it is not energized, the shoe on the locomotive causes an electropneumatic valve aboard the locomotive to begin exhausting air from a small reservoir through a pipe into the cab which has a shrill air whistle attached to it.  The Engineer then pushes a button or moves a lever and hears a bell ring, which forestalls any further air exhausting through the ATS whistle.  If he does not acknowledge, then the ATS reservoir is depleted, a slide valve moves and exhausts brake pipe air at a service rate.  This is called a penalty brake application, and it will stop the train.

So, really, ATS only checks to see if the Engineer acknowledges passing a signal that is less favorable than green.  It does not require him to slow down -- it only assures that he is aware of the signal indication or permanent slow board.

We used to have ATS on the AT&SF Topeka, Emporia, and Ark City Subs. Back then we were still using the SDP40f's. They were equipped with ATS. Most of the time it worked OK, but once in a while, it would malfunction, and you would have to cut it out. If I remember correctly, when it gave you a penalty reduction, you had to come to a complete stop, because the re-set button was outside on the frame of the locomotive. On both Subdivisions, trains we allowed to run 90 mph with the ATS cut in and working properly. If it was not, you were limited to 79mph. When it was cut out, the DS had to establish Absolute Block in advance of your train. A few years later, we started getting Red Rock coal trains off the UP at Topeka, and would handle them as far as Ark City, KS usually with a UP unit on the point. Those units had a cab signal system, and sometimes the system would react when it went over one of the inert inductors for the ATS system on the Topeka Sub, if it was not cut out properly.