A loose trackwork steel plate is suspected of causing a train derailment in France that left six people dead and injured nearly 200 others.

The packed train, which left Paris with 385 passengers, jumped the rails about 20 minutes into a scheduled three-hour journey as it travelled through Bretigny-sur-Orge station.

It ploughed into the platform with several carriages overturning. 

Human error has already been ruled out, according to Transport Minister Frederic Cuvillier.

Attention has focused instead on a set of points, which switch trains from one track to another.

Investigators have found that a steel plate which should have remained bolted in place moved instead to the middle of the track junction, preventing rolling stock from passing through, according to train operator SNCF. 

"The reasons why this fishplate dislocated itself is the very focus of the investigations," SNCF head Guillaume Pepy said.

Checks on some 5,000 similar junctions throughout the French rail network would start immediately, he added.

"The SNCF considers itself responsible," said Mr Pepy. "It is responsible for the lives of its clients."

The train was about 20km into its 400km journey to Limoges.

Passengers and officials in train stations throughout France held a minute's silence at midday to commemorate the accident.

Hundreds of thousands of people are expected to take trains this weekend for the coast, mountains and to see family.

Summer weekends are always busy on France's extensive rail network, but this weekend is typically one of the busiest since the country celebrates Bastille Day tomorrow.

Local officials say the final death toll may not be known until an overturned train carriage is lifted and all the wreckage examined.