Well, the first thing we nee to know is: Which part of the underground do you ride? My experience is that the underground has a rich vocabulary of smells (good and not-so-good), and many of them are location dependent. read more
If, however, the smell you're experiencing is the one I'm thinking of, it's probably burning gunk - the incredibly unpleasant mix of grease, dirt and metal filings that gets on pretty much everything in the underground, especially around track level. read more