German passengers on a runaway tram took their lives in their own hands to avert disaster on Sunday morning, breaking into the unconscious driver's cabin to activate the brakes.

The line-66 tram to Bonn had left Siegburg at 00.30 local time, but worried passengers called for help after it started to hurtle through stations without stopping because the driver had fallen unconscious - and pulling the emergency brake didn't work.

"We feared for our lives. We couldn't do anything," passenger Manfred Daas told local newspaper General Anzeiger. 

Telephoning the police for help, passengers were connected to the transport operator SWB. Under instructions from an SWB technician, two men forced their way into the driver's cabin to safely halt the tram. 

Police said the 47-year-old driver was taken to hospital for medical tests, while the men who broke down his door were treated for minor hand injuries. 

After the rescue, the investigation

The mayor of Bonn, Ashok Sridharan, praised the passengers, saying "They did exactly the right thing in a dangerous situation and probably saved lives," but attention has now turned to how the situation arose.

"According to witness statements, attempts to pull the emergency brake lever did not slow the train,” local police said in a statement. SWB explained that the emergency brake does not operate automatically unless the tram is already stationary, for instance if a passenger is caught in the doors. In other circumstances it merely alerts the driver, who then stops the tram as soon as it is safe to do so.

SWB also said that certain systems are designed to kick in if a driver appears to be unresponsive, but they seem not to have worked in this instance. The transport company defended the decision not to cut electricity to the tram, which would have brought it to a rolling halt, explaining that breaking into the cabin was a quicker solution.