Patrons reported that a male was acting erratically on a train, and that he got off at Powell Street Station. Officers responded and were told that the man had entered the #2 track, walking toward Montgomery Street. Trains were held out and power was dropped. Officers entered the tunnel and located the man on the #1 side. He was detained and moved to the platform. Trains were released after a ten minute delay in service. The man told officers he entered the tunnel because he was ill. The man did not meet the criteria for psychological evaluation. He was transported to SF General Hospital for treatment of an apparent allergic reaction to medication.