The high-speed TGV train entered a bend of an older track at a speed of 265 km/hour instead of 176 km/hour, the French railroad operator SNCF said on Thursday after conducting an investigation.
The company said the train began breaking a kilometre too late as it approached the older section of track 20 kilometers (12 miles) north of Strasbourg. The error caused the next generation train to strike a bridge, derail and break in two, killing 11 people and injuring 42, including four children.
It was the first fatal accident on France's flagship train service since it was launched in 1981.
The train was testing a run between Paris and Strasbourg on Saturday as part of a high-speed line connecting the French capital and eastern France, expected to go online in April 2016.
Investigators found seven people were in the driver's wagon instead of the authorized four. Passengers included engineers and staff, as well as family members.
The company said it would further investigate why four children were on the train against company safety regulations and would discipline those responsible for the crash.
cw/jil (AFP, AP)