THE casualty toll in Belgium from the derailing of a train carrying highly toxic chemicals -- which exploded and sent spectacular strips of fire into the night sky -- has risen dramatically to one dead and 49 injured.

Two victims were in intensive care and three of the injured were rescue workers exposed to fumes from chemicals that spilled from the train that derailed near the city of Ghent, officials said.

The accident and blaze happened around 2am (1000 AEST Saturday) and prompted authorities to evacuate around 300 people from their homes.

The victims were people living well away from the scene of the accident, and Interior Minister Joelle Milquet blamed toxic fumes from the highly flammable liquid chemicals for their injuries.

Six of the train's 13 wagons derailed and two were left lying on their sides, said Infrabel, the state-owned company that operates Belgian railways.

The blaze led to a series of explosions in the railway wagons, then a spectacular strip of fire spread over hundreds of metres prompting authorities to evacuate residents living within 500 metres of the scene of the accident.

The train was transporting the toxic chemical compound acrylonitrile, which is used in the making of plastics, officials said.

Exposure to acrylonitrile can cause headaches, dizziness, nausea and mucus membrane irritation.

Milquet said toxic fumes reached much farther, via the drainage system, than the 500-metre perimeter that was set up.

"Some of the chemical product went into the drains and caused a kind of chemical reaction with gases that are toxic and escaped into certain streets beyond the perimeter that had already been evacuated due to the fire," she said.

Firefighters let the wagons burn out in a controlled manner as water could have released further toxic chemicals.

The causes of the accident remained unclear. The cars derailed as the train changed tracks. The train driver said he had been travelling faster than the speed limit for the area.

The train came from the Netherlands and was bound for Ghent's seaport.

Train services between Schellebelle and Wetteren were disrupted and problems were expected for two days, with buses laid on to transport passengers.

Two similar accidents involving trains carrying tanks of toxic products have occurred in Belgium since May last year.