Use a Dremmel tool with wire polishing bit to polish contacts
Use high temp disk break wheel bearing grease on the gears, do not use oil.
Apply one drop of oil to the end of shafts where thrust washers are. No more!!!
Can use baking soda with super glue to repair cracked gears because there baking soda causes the got to cure instantly. Fill a crack with solid baking soda. Add super glue on top. The baking soda makes cyanoacylate glues harden instantly.
Clean track with Goo Gone, never steel wool (wears it down).
Nickel-sliver conducts power better
Brass tarnishes and loses power conduction. Some people like the way is looks.
Code 100 - taller, most common. Not realistic. Runs any HO trains, even old, poorly made wheels.
Code 83 - More realistic lower height, similar in scale to main branch track.
Code 70 and 55 - Less useful for older trains. Most realistic for light rail scale.