About 60 suffered minor injuries.

A city transport spokesman said a speeding train entered Christophstrasse station at 1740 local time, failed to stop and crashed into the back of another which was stopped at the station.

The driver of the tram which failed to stop was among those with critical injuries.

A power cut following the accident brought the whole Cologne region rail system to a halt.

Protoype

According to the spokesman for the tranport undertaking, the train which rammed the other was a prototype still under test.

It is supposed to brake automatically when another halts in front of it.

The train is thought to have gone through a red signal about 200m from the station. It was travelling at about 50km/h when the collision happened.

In investigation into the cause of the accident has begun.

Rail workers took several hours to separate the wreckage of the two trains.