Darrell, for many years I thought the same thing. This is why I didn't believe Ed Rappe on that other thread when he said it wasn't AC. However, I could not find any official source that says it is AC to refute him. Would a regular 60Hz sine wave have a common reference point that is held to zero potential?
Now when it says above that negative voltage would be an undefined value doesn't make sense to me because in the early days of DCC systems there was a way you could control one conventional locomotive with a DC can motor in command mode. I believe the system did this by changing the lengths of the top of the square wave but if you wanted to go in reverse than it changed the lengths of the bottom of the square wave so it had to be getting reversed polarity voltage to go in the opposite direction. I guess it wouldn't be negative voltage just positive voltage but in the opposite polarity.
Then when it says "the rails are alternately energized" this would also indicate to me that it is AC. I don't know, I am not an electrical engineer. The whole thing is very confusing to me. Maybe I was correct all along and it is actually AC and Ed Rappe got some misinformation? It would be cool to clear this up once and for all.
