The unexploded firebomb was discovered on a railway track in a suburb south of the German capital, the BBC reported.
Two devices were found Monday, one of which detonated on cables critical to the Berlin-Hamburg rail line, Der Tagesspiegel reported. The explosion caused extensive damage along the track between Brieseland and Finkenkrug stations, including knocking out signaling.
The second device, which consisted of seven bottles filled with flammable liquid and a fuse, was disarmed.
A leftist group called the Hekla Reception Committee claimed responsibility for the track attack in an e-mail to media outlets. Among other things, the group said it was protesting the 10-year war in Afghanistan and was calling for the release of U.S. Army Pfc. Bradley Manning, jailed on accusations of leaking U.S. military information to WikiLeaks.