To add to the above. Left alone, the drivetrain accumulates a film of waxy/dirty oil residue, which is an effective rust preventative. New stuff and stuff that people religiously keep clean lacks this nasty film and is more pone to rust.

In any case, minor surface rust isn't a crisis, and is rapidly burnished off the working areas the same way trains keep the tracks polished. However don't let rust get deep on the surfaces of the chain plates as they're thin to begin with, and don't have much reserve strength. (minor rust around the plates' rims is OK).