Discovered a tiny rust spot on my trunk about five inches above the I in Infiniti. Took it to the dealer and they agreed to repaint it under warranty. The service manager said it was likely due to rail dust.

 

Getting mine repainted next week, rust under the chrome spoiler and above the right tail light.

 

Rail dust is tiny deposits of ferrous particles (commonly from cars transported by train... hence the name, but also from airborne pollutants, brake dust, etc.) that sit on top of the paint. I have never personally heard of or seen a case where this has led to actual damage of the metallic substrate.

Rail dust is actually very easily removed as demonstrated here...
How to Remove Rail Dust with CarPro IronX and a Clay Bar by Addison Good

Zach McGovern
Detailed Image Ask-A-Pro Blog Author
Attention to Detailing - Peoria, IL