leds and capacitor

Bi-polar caps won't help at all.  A rectifier and a 470uf cap should remove the flicker unless you're drawing a LOT of current.  LEDs, by their nature, are DC device, so they only light on one half of the applied AC, even if you didn't have the diode.

Bi-polar caps will screw up the ERR back-EMF speed control.  For straight TMCC with the DCDR, a low value bi-polar, say 1uf at 50V might knock down some brush noise.

 ..the rear Oliver's on the a units...

Sorry, but what is an "Oliver"?

I'd like to ID the Oliver as well.

Arthur & John, thanx for asking about Oliver!  Thought this thread was getting overly complex!

Blame it on the cat!

I do so hate auto-correct, I have gotten some "interesting" words at times.

Fred, adding a capacitor can certainly help that, however, the value and the voltage rating of the capacitor needs to be selected very carefully. You can add a capacitor with voltage rating of at least twice the voltage it’ll be operating on.  Now, choosing the value of capacitance is that hard part. Here’ll you’ll have to know exactly the frequency you’ll be operating on which will then help you choose a capacitance in order to smooth out your power signal. Please also note that the capacitor should not be a polar one since they are mostly used in DC circuits for noise reduction. 

I have no idea what you're talking about here, but you seem to be making this a massive engineering exercise.  Also, what's a "polar" cap? 

Again, for this task, the polarized capacitor I specified in my original post is fine, and there is no place for a bi-polar cap in this discussion.