A sergeant responded to the station regarding a suspect lewdly exposing himself to juveniles while aboard a train arriving at the station. On the train’s arrival, the sergeant detained a subject who matched the suspect's description given earlier by a reporting party. A wants’ check of the subject revealed he was not in compliance with his sex-registrant status, so he was placed under arrest for that violation. A search for a victim of the indecent/lewd exposure was unsuccessful, and the reporting party/witness was not readily available for an in-field identification. The sergeant debriefed the subject before he was booked into the county jail for not being in compliance with his sex-registrant status. Further follow-up with the witness will be performed.