Atlas basic 3-rail track sections (and flex) are fully isolated -- i.e., the running rails are insulated from each other as well as insulated from the center rail. So you can use them for either 2-rail or 3-rail operation (as I've done on my work bench). By the way, MTH ScaleTrax is also insulated this way.
The exceptions are Atlas 3-rail turnouts and the Atlas Terminal Straight where there is some "through wiring" and some connections between the running rails. The ScaleTrax turnouts I have are fully insulated.
My recollection on the LokSound DCC decoder is that it will operate in conventional DC.