The chugger was one of the very first attempts at on-board steam loco sound.
The "chugger" is a coil which shakes a couple loose washers when energized
(poor description) that sound just a very little like a steam loco's exhaust
"chuff". I recently restored a 1936 #385 for a friend and the circuit in
his was that a lead from the "hot" terminal on the top of the motor assembly
(where the head light lead is also connected) went to the chugger on-off
switch on the boiler backhead. The switched power from this switch then
went to one end of the chugger coil. The other end of the chugger coil went
to the insulated contact finger on the cam assembly on the forward driver
axle. Thus when the backhead switch is "on", the chugger coil is grounded
(and thus briefly energized) each time the driver axle cam passes the
contact finger, making a speed dependent "chuff" noise. Hope this helps.
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