I'll weigh in too- in my case, I compare with a brand new motor brush, and if it looks significantly worn down, I'll probably replace it. If it's anywhere near half the length of a new brush, it's in dire need of replacement- but other than that, personal preference. I have found that some trains, which didn't have all that much wear to the brushes, would perform noticably better if I replaced the brushes. I've also kept using brushes that were somewhat worn, and found the performance of locomotives to be a-ok.
I will say this- I always, always replace the brushes AND brush springs for whistle tenders. Why? Because it makes 'em work better, and the more efficiently those motors run, the less they're going to slow down the engine when you activate the whistle.
That's my thoughts- I will say, if you have a diesel locomotive for example, which runs better one way than the other, replacing the brushes is sometimes the solution.
Wayne (Flintlock) makes some excellent points- it always pays to clean up the armature. Swab up all the carbon dust with a Q tip and 99% alcohol (or whatever is the highest percentage you can easily procure), and if you want to polish up the plates, pencil erasers, track erasers, and fiberglass pencils are all very helpful. Take a toothpick and carefully clean carbon dust and gunk out from the gaps between the commutator plates.
I also like to use a Q-Tip and alcohol to clean out the inside of the 'tube' that the brush sits in, aka the brush holder.
At the end of the day, unless the brushes look brand new, replacing them can’t hurt. But unless your motor is running really rough and drawing way too much current, you'll be ok keeping the old brushes- just remember to clean them before you put 'em back in after cleaning up the motor
-Ellie
