One passenger suffered smoke inhalation, but no one else was injured, initial reports from police in the German state of North Rhine-Westphalia said.
As the metro departed from the underground Kaiser-Wilhelm-Park station there was a loud noise and the train ignited. Metro personnel were able to help passengers escape the flames quickly, and a lucky breeze from outside cleared smoke from the station, a police spokesperson said.
Officials plan to reveal more information at a noon press conference.
