It's not so much the voltage, it's the amps that can cause trouble. I would stick with DC. But I believe a standard O gauge transformer puts out a max of 14 to 20 VAC for accessories. The water tower my require a higher voltage, AC or DC. if indeed it is 30 milliamps. That's a lot of amps.. I would test out hooking up to transformer on the AC side and see how it works. Just touch the pos & neg wires to the transformer just to be on the safe side...
Good luck'
