Throw a diode in, that cuts the power to what it would see in a TMCC locomotive, it only passes one half of the waveform. For fine tuning of the smoke volume a bit lower, add diodes, each additional diode will drop the voltage about .7 volts. I use this technique for a variable smoke regulator board. In my case I used bridge rectifiers for reasons that will likely go a bit over your head, but just single diodes are the ticket in your case.
