The signals run on AC, but if you run them for any length of time from 18VAC, they seem to die. We had seven or eight of them on our modular club layout, all but one or two are in a bag on my workbench with busted electronics.
Of course, MTH says between 10 and 18 volts, but don't you believe them! These were all running from a PH180 brick, and I know they're not getting more than 18 VAC.