Short version: There is a lot of debate online about whether or not to call DCC AC, most of it based on "no true Scotsman" definitions of AC. My take: Current flows in both directions, switching with a frequency determined by the signal [i.e. the pulse width - short for 1, long for 0]. In other words, it alternates. It has a phase. I would call that alternating current, even though it's not the fixed frequency sinusoidal waveform familiar from household AC. But whatever you want to call it, it is not DC because, again, it flows in both directions alternately.
Long version: I've read the DCC wiki. (Some of the things in it are just flat out nonsensical, written by somebody with apparently little knowledge of electrics/electronics.) Here are some of the things it says in the Myths section. I added some italics for emphasis to a couple of things it does get right.
- DCC uses a Differential Signal on the rails. FALSE. While the NMRA DCC Standard describes the signal as a Differential Signal with no ground, this is only a description of what the waveform looks like on an oscilloscope, not the signals themselves.
- There are Positive and Negative voltages on the track. FALSE. As it is binary, only positive voltages present. One rail is energized while the other is held to 0V (Ground). The two rails are always opposite in phase/state. When the phase change happens, the relationship between the two rails is inverted. The direction of current flow changes based on the state of the rails.
The statement "only positive voltages present" is so electrically illiterate it is not even wrong (to quote the great Wolfgang Pauli).
In the footnotes to that section it also says "As DCC is Digital, the only valid states of On and Off, ON represent [sic] by the presence of a positive voltage, and Off being the lack of voltage." Again, this is nonsense: DCC is a constant 14V (in order to provide constant power to the locos and accessories), the binary state being indicated by the width of the pulses, i.e. the voltage direction changes. Whoever wrote that is apparently under the misapprehension that a binary signal is by definition or or off. There are many ways to encode a binary signal, and DCC uses pulse width.
It also says "The NMRA's DCC Standard states that the rail considered positive is impossible to define". And then bizarrely, in the footnote to that very statement: "The NMRA definition of the positive rail is the right hand rail as seen from the engineer's position in the cab." [Italics in original.] I suppose you could define it that way if you wanted, but if you did then the left hand rail must be alternately 0V and +28V in order to reverse the current flow. But more importantly, these two statements cannot both be true at the same time!
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