Well, long story short.  I have connected the function 5 wire from the decoder to the NMRA DCC connector pin3 and programmed the CV with the basic On/Off function.  I leave the switch behind the boiler plate on so when I choose F5 on the cab, it smokes!!  A full 14V is applied through the switch while only drawing 3.3 mA through the decoder.  This is then regulated to 12V at the smoker while drawing the full 117 mA reported earlier.  I can only assume Bachmann had the foresight to put a relay in there somewhere. 

(This is usually the part of the story when someone finds a wiring diagram that was buried away which easily proves my reverse engineering.)

So, there you go MGB.  If you feel like trusting a guy who wears black socks in sandals, hook up a function wire to pin3 and smoke away....

This is sort of fun as long as something doesn't melt.  I'm sure a lot of you guys will cringe at my method but here it is. 

I built a quasi-dummy plug like I described, hooked up analog DC and got the loco to run but no smoke.  I followed a hunch regarding my trace and decided to short NMRA pin 3 to 4/5 (left rail/ left motor) and voila...smoke!...but only while running "forward".  Kevin, you were right about the voltage regulator.  At 18V applied, the smoker was limited to 12V and 117 mA.  There was nothing in "reverse".  I might only assume polarity is going to have something to do with this? I checked continuity with the smoker between both the motor and track but there is no continuity anywhere. 

I then decided to try this same "short" with the DCC between the Pin3 smoke and Pin4 track.  It only sort of worked but again only while running "forward".  The system was giving me a track voltage of 14.4V with the meter set to AC volts as described in the installation test.  (Is this so that the meter can read both the "top" and "bottom" of the DC square wave?) At the smoker it showed 13.3 V but didn't smoke anywhere as nice as with 12V analog DC.  I reset the meter to plain old DC Volts and it gave me 6V.  Is this because only the "top" half of the square wave is seen and being used to heat the unit?

My decoder has a Function 5 wire that I could hook up to Pin 3 but it is limited to 100mA.  I can only assume that the decoder is wired to supply the full 12-14 track Volts but I am still short of the 117mA measured.  And yes, according to the manual the total amperage draw is cumulative. And besides being a neat option, I don't have any real desire to run smoke with the decoder.  The front switch would work fine and perhaps preferable.  But how now do I get full voltage to that Pin3?

Yes the switch was turned on.  Yes there is oil in the smoker.  Yes, the heater wire is continuous and not burnt out.  No, I have not tried a dummy plug.  The locos came with decoders already installed.  It looks like a dummy plug shorts pins 8, 1, 2 (right rail, right motor, rear light) and pins 4, 5, 6 (left rail, left motor, front light)?  I'll build one when I get back home and try it out.

Kevin, everything you mention makes sense.  If I remember right, when I measured voltage at the smoker with the decoder installed it was zero.  But it should have been 12-14V track power if it was coming from rails, even if standing still right?  I don't have stationary rollers (yet) to test it at motor speed.  But even if it is connected through the motor, shouldn't there be some minimal voltage even when standing still?

To add a twist...I traced the board that contains the switches behind the number plate.  The smoker switch is conected to red/ brown (connector posn 1,2  NOTE: this is the white board connector NOT the NMRA connector) while the smoker itself is connected to yellow/ grey (connector posn 4,8)  When I traced the red wire, it appears to run directly to the NMRA connector pin 3, which shouldn't be and isn't connected to anything according to the installation.

I sincerely appreciate the suggestions.

Hmmm...no replies.  I have the exact same issue.  Soundtraxx DCC/ sound decoder wired through the 8 PIN connector as detailed.  F5/ F6 wires not connected to anything.  Smoke switch is turned on but nothing.  I have metered the smoke generator "coil" and it is continuous.  I have check voltage at the generator, at the points, at the board and at the switch but there is no DC voltage showing up anywhere.  Does this thing need a rectifier off track AC?  That would suprise me since everything else DC is running.  What is the routing for this power?  Is there something buried in a CV that I need to turn on?  Can anyone help?