Rust is iron oxide, so if it is truly rust, it is not chemically attached to a brass token. Brass tokens can have stuff on them that will come off with a toothbrush and hand soap, or it may take a few weeks' soak in olive oil to release it. If it is still there, there are other methods to clean up the surface, but I would recommend not using them without first determining whether the token is just a common one or is rare and/or valuable. If it is the latter, and you (or your estate) ever want to sell it, the prospective buyer might not appreciate it being cleaned. Remember tokens and coins can always be cleaned, but any damage (in the eyes of a prospective owner) caused by that cleaning would be tough to restore. Post a pic of what you have!
John in ID