What decideds which resistors and where they go?

[Jkchubbes]

Admittedly I don't know much if anything about electronics and on the couple installs I have done I have always just copied what someone else has done.  On my work bench right now I have two ScaleTrains SD40-2s that I am hardwiring an LM3-S into.  I am using the factory LED light boards and also installing two ditchlights with two 0402 SMD LEDs.  On my previous LED installs I used the supplied resistors, one on each LED lead.  Recently it was suggested to instead install the resistors on the blue common wire.  How is this different than being on each LED wire, it seems easier?

[Alan]

Do not do it that way. Use a separate resistor for each LED.

With a single resistor on the blue wire each individual LED brightness will fluctuate depending upon how many LEDs are on at any given time. Additionally, the LEDs will not dim in a predictable manner due to minute differences in LED manufacturing. The headlight will go bright dim bright dim when you activate the ditch lights. Not good.