Helpful post at another site on this issue is elucidating. The meaty part of the post reveals that SMART does not use the GPS-based PTC system but a circuit based(rails) ATC system called E-ATC. The E means enhanced. ATC means Automatic Train Control, Berg.

So it is actually a form of the ATC that Hotwater described above. But it also meets PTC requirements.

So this could be an example of ATC and PTC getting a little overlap and sharing a label, because SMART refers to their own system as PTC in their materials. Not the technical materials but like newspaper articles and stuff.