…and I promise to write it down this time: For an LED in the front of a diesel, given that the longer lead is positive (red wired the way I do it), the positive lead goes on the left side wheel pickup, to obtain lighting moving forward, correct? Therefore, when wiring another LED for the rear end, the positive (red wire) is now wired to the right wheel pickup, to obtain lighting when the diesel moves in reverse, correct? Cedarwoodron
That sounds right and as long as your track feeders put the +DC on the left wheels running forward and you use the proper value resistor it will work, for DC track power. If you are running with DCC both LED’s will light regardless of the loco’s direction.
Your theory is correct but you will be burning out a lot of LEDs without additional components to limit the current and maintain the proper polarity in both directions.
I generally use a 470 ohm resistor for each LED, soldered in-line with the positive lead. Cedarwoodron
The “standard” is that an engine will move forward with positive voltage on the right hand rail when facing forward.
That’s not to say you might have things wired in reverse. Your simplest approach would be to wire up a single headlight and see if it works as expected - if not, reverse the leads. Wire the other end the opposite as the first. That probably would have taken less time than posting your question in the first place !
NOW - write it down so you don’t forget ! 
Mark.