A fast-moving passenger train late Sunday slammed violently into the back of a slow-moving freight train that was travelling on the same track for reasons that remain unexplained.
Authorities said lightning and flood damage caused by heavy storms that affected all of western Europe in the past few days were being explored as a possible cause.
Investigators were on the scene, but a spokeswoman for the prosecutor's office in Liege warned that finding the cause "would be difficult".
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Nine passengers were injured to varying degrees with some described as being in a critical condition by authorities during a press briefing near the scene of the crash.
There were roughly 40 passengers on board the train that was travelling at about 100 kilometres (60 miles) an hour at the moment of the collision.
Investigators believe the passenger train rammed into the freight train that was travelling at about 10-15 kilometres an hour, said Brigitte Leroy, a spokeswoman from the prosecutor's office.
The train was travelling from the west to the east of Belgium on the Namur-Liege line when the accident occurred in the municipality of Saint-Georges-sur-Meuse at 11:00 pm (2100 GMT).
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The crash killed the driver of the passenger train, authorities said.
"The passenger train is really in a bad way, it's stunning," said Francis Dejon, mayor of Saint-Georges-sur-Meuse who had been following rescue operations.
"The front carriage is scrunched back up on itself. We were very lucky not to have more victims," he told the Belgian news agency Belga.
"Two of the six carriages derailed and are lying on the tracks," the Infrabel railway infrastructure company and the National Railway Company of Belgium (SNCB) said in a joint statement.
Witnesses told Belga news agency the collision had been "very violent,", with several passengers having to be extracted from the wreckage.
Frederic Sacre, the spokesman for Infrabel, said investigators would look carefully at a report of lightening on the tracks about 90 minutes before the crash.
The SNCB rail company on Sunday evening said lightning had caused signal problems near the site, but that the incident was over.
"The investigation will tell us if there is a link with the recent storms," said Leroy, of the prosecutors office.
"There was a lot of rain yesterday (Sunday), other train lines were completely flooded," she said.
Belgian Prime Minister Charles Michel was to visit the scene with Belgium's King Philippe later on Monday.
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"All my condolences for the families of the victims... and a fast recovery to the injured," Michel said on Twitter.
A crash in 2008 took place almost at the exact same location, in which 42 people were injured. That incident was later blamed on human error and a power failure.
In February 2010, 18 people were killed and 95 injured when two trains collided in a Brussels suburb in one of Europe's deadliest railway accidents of the past decade.
More recently, one person was killed and nearly 50 injured when a train carrying highly toxic chemicals derailed and exploded near the city of Ghent in May 2013.
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