If it can run on 18 VAC, then they need to update the user manual for the locomotive module.
WARNING: Maximum voltage is 24 Volts. Only power your LM-3S-G module
with a Ring Engineering Inc. RailPro Power Supply designed for the LM-3S-G
or a DCC power supply that only has the signal that is described in NMRA DCC
standards S-9.1 and S-9.2. You may use a proper battery to power your LM3S-G.
The manual doesn't state dc or ac. However I verified with Tim Ring that ac okay. Also the railpro user group forum discusses this.