An unemployed skateboarder who was threatening to blow up a station in Fulda, in central Germany, was described by the authorities as "muddled and motiveless".

Police say he has no connection to blackmail demands made after a spate of vandalism on the country's train lines.

$6m blackmail demand

Signing themselves the 'Friends of the Railways', blackmailers have written four letters demanding $6m from the German rail company Deutsche Bahn.

Police confirmed that the group caused three recent train accidents in the northeast of the country, and precautions against the extortionists are being stepped up.

In the latest acts of vandalism, screws were loosened on railway tracks.

And a train narrowly escaped derailment last weekend in Berlin when it went over concrete blocks placed on the line.

Deutsche Bahn says it is trying to guarantee safety. But with the busy Christmas travelling period coming up, passengers are understandably nervous.

Tornados on standby

On Monday, Tornado fighter jets fitted with surveillance cameras were deployed to guard tracks across Germany.

Helicopters are also being used to keep guard and the armed forces have been drafted in as part of increased security measures.