Thank you for the answers, in a simple form. :-)
A follow - up question : How is it possible to have ATS - equipped passenger trains AND non - ATS equipped freight trains operate on the same line segment ??? I'm guessing it's because the signal system itself continues to function with or without the ATS shoe being present ??? As I understand it, with an ATC system ALL lead locomotives MUST be so equipped. I also recall that even though U.P. and C & NW both used a ATC / cab - signal system, the 2 systems were not compatible, which forced a home road engine to lead. Why not ONE system nationwide ??? It would seem that all the current fuss over "The Great New Positive Train Control System ", and all its' safety applications, was already addressed many years ago with ATS & ATC systems.