James Helm reports: "The train's first fatal accident in its history"

German prosecutors are investigating reports that the monorail train hit maintenance equipment left on the track.

The train jumped its tracks about 50 metres from a spot on the track where maintenance work was being carried out under a renovation programme begun in 1997.

Witnesses said rescue workers pulled more than a dozen passengers out of the Wupper river. The train driver is among the injured. Three people were killed outright, while another died in hospital later.

Wuppertal Railways spokesman Lutz Meyer said: "It is the first accident in the 100-year history of the suspension railway ... we are really shocked."

Lutz Meyer of Wuppertal Railways: The first accident in 100 years of operation

The accident happened at the beginning of Monday's rush hour when the train was crowded with commuters.

The train was the first to run on Monday morning after the track was closed for the weekend because of the maintenance work.

The Mayor of Wuppertal, Hans Kremendahl said he suspected the accident may have been caused by a metal clamp which should have been removed from the rail before trains began running again.

The BBC's Terry Stiastry: "It's a very unusual train service"

The train, called the Schwebebahn, carries 70,000 passengers a day and has until now been regarded as one of the safest and most popular means of transport in the region.

It is suspended by distinctive steel girders from a monorail above the roads and rivers of Wuppertal.