NMRA Standard S-9.1 Most definitely describes it as a AC signal.
And I Quote "Digital Decoders shall be designed to correctly decode signals with transitions whose slope is 2.0 volts per microsecond or faster across the voltage range from -4 volts to +4 volts."
This clearly shows Alternating Current, i.e AC
And yes, the rectifier would convert to signal to flat DC anyway so there should be no issues running the lionchief loco's.