A patron reported, via a transferred 911 telephone call, that a man had walked off the north end of the 16th & Mission platform into the tunnel leading to Civic Center station. The suspect was last seen on the catwalk adjacent to the M1 tracks. A sergeant and four officers responded. Power to the tracks was dropped. Officers proceeded to search the tunnel on foot because one officer was advised that the suspect appeared to have been trying to touch the electrified-third rail. Officers located the subject lying down in the tunnel next to a cross-passage door, adjacent to the M1 track. The area was searched and nothing suspicious was found. The man was taken aboard a train and transported to Civic Center station. The subject could not carry on a coherent conversation, or explain why he was in the tunnel. He was determined to be in need of a psychological evaluation and was transported to San Francisco General Hospital. Train service was delayed for 36 minutes. The TSOC was notified.