With S Scale smoke units I usually turn them off and on with a relay off the decoder instead of the smoke unit off the decoder. Much less amperage draw on the decoder. You can then power the smoke unit with either track voltage or reduce it using a linear or switching (cooler) voltage regulator of either fixed or variable nature. If you make it such that the relay is NC then you'll also need a hardwired switch to isolate the smoke element for Programming Track decoder programming or the smoke element will appear as a short and prevent CV programming. If you prefer the relay NO, then no need for the hardwired smoke on/off switch.
Dave
