ATHENS A new extremist group has claimed responsibility for a bomb that exploded near the offices of Hellenic Train, Greece ’s main railway services operator, and the planting of another near the Labor Ministry in early February. The explosion on April 11 evening resulted in limited damage and no injuries. The perpetrators had forewarned of the explosion by calling two media organizations about 40 minutes before it happened. In a lengthy posting on a website late on April 13, the perpetrators,