The group may not be known to the general public, but it is known to the German security authorities:
The group “Kommando Angry Birds”, which has claimed responsibility, is “known to our authorities as a left-wing extremist join-in campaign”, said Interior Minister Reul. The group had “already sabotaged the Düsseldorf area several times in recent years”. The attack shows “how vulnerable our critical infrastructure is”.
Translated from: https://www.sueddeutsche.de/wirtschaft/bahnverkehr-anschlag-auf-bahnstrecke-linksextremistisch-motiviert-dpa.urn-newsml-dpa-com-20090101-250801-930-864123
From the same source:
A letter of confession was published on the left-wing platform “Indymedia”. In it, the “Angry Birds Commando” claimed responsibility for the crime. The authenticity of the letter is still being investigated, a police spokesman said. According to the police, the two incendiary devices on the central railroad line had the same design.
Regards, Volker