@gunrunnerjohn posted:... and it has two FET outputs that can support plenty of current to manage the lights, and a couple more PIC processor pins that could drive a flickering LED with no problem. It's so much easier to do stuff like this if you have a little intelligence in the circuit.
Only 2 FET outputs?! 
How about 4 FET outputs! 
But seriously though; GRJ has it right. IF you can find the self-described "needed help" to go down the Arduino or microcontroller path, many doors open.
Like his Chuff Generator, above custom-built board uses a custom-programmed 8-pin microcontroller IC chip (less than $1). I figure the brake sparker could use up to 4 independent flickering LEDs; those would take 4 of the 8 pins. You need 2 pins for DC power. So that leaves 2 pins, one of which can sense the flywheel striping on MTH PS2/3 engines like the Chuff Generator.
As GRJ says, this gets you an accurate and real-time measurement of actual engine speed rather than some proxy (e.g., speed derived from applied motor voltage). Then literally do-the-math to calculate changes in speed...and when the deceleration exceeds some threshold then independently flicker the 4 LEDS.
Here's the 4 LED board programmed to independently flicker 4 LEDs that simulate the 4 50-cal guns on a 1:48 scale model of a P-38 Lightning. Like the brake sparking application, the LED action is synchronized to an external event...in this case when Major Bong's Marge comes in for a strafing run.
If just getting started into microcontrollers I suppose the Arduino might be the way to go. Not sure what to suggest. That is, something like the Arduino-Nano ought to fit in a O-gauge diesel shell. Might still want to add FET output buffers to insure adequate current drive to flickering LEDs...for example you might be driving multiple LEDs on each of the 4 outputs if sparking is seen on either side of the engine.
