Assuming someone has been validly excluded by Trimet (including service of a notice of exclusion with a right to a hearing to challenge the exclusion), a person could potentially be charged with the crime of trespass, which is a misdemeanor. Depending on criminal history, the circumstances of the particular case, and bad luck, someone could be sentenced to serve some jail time for trespass (although most trespass cases in Portland do not involve jail time). The trespass statutes are in Oregon Revised Statutes chapter 164, if you want more information on this.
Good luck!