A bomb ripped through a crowded passenger train heading for the southern city of Karachi early today, killing eight people and injuring 25 others, Pakistani railway officials and doctors said. The train was pulling out of the station in Hyderabad, about 100 miles north of Karachi, when the bomb went off about 7 a.m., they said. Four passengers died in hospital, said the railway workers. Hospital officials said another four people died within an hour. There was no immediate claim of responsibility for the explosion, and Pakistani police say they have no suspects.