The German security services managed Monday to foil an improvised bomb attack on the central subway station here. The railway workers detected a suspicious package at the subway this midday and promptly called in the police and explosive experts who managed to defuse at the last minute the device consisting of seven bottles with flammable liquid. The foiled arson attack bore the hallmarks of a left-wing group called Hekla, Federal Police spokeswoman Nadine Marschner told reporters here. The group earlier attacked a utility shaft of a high-speed link to Hamburg where the train traffic temporarily came to a halt.  here