I wondered why this semi-old thread had been revived. Units like those that TAS produced are still made somewhere because 3rd Rail still use them. But where you might get your hands on one is hard to say although they do come up from time to time on the auction site.
But based on my own experience with these units, I think that while it may be the case that a component on the PCB board has failed that seems unlikely with an engine that has no run time. I wonder about your switch and whether you are getting any power to the resistor - you mention checking continuity but what about voltage at the resistor terminals?
I had a look at a spare 3rd Rail smoke unit I have and it was clearly designed to be controlled by TMCC functions and is otherwise “dumb” with no logic on the PCB. So another option may be to get an old Lionel TMCC smoke unit and try that.
Hopefully one of the experts who knows exactly what components are on the PCB can chime in about whether one of them might need to be replaced. A photo of your unit would help.
