Officials said the train carrying 300 passengers from London to the seaside city of Plymouth slammed into the car at a crossing on Saturday.
"We are unfortunately in a position to confirm six deceased patients, 11 serious patients," ambulance official Graham Groves said.
The official said a further 25 less seriously injured patients were removed by ambulance for medical care.
Search and rescue officials described a dark scene of shocked survivors, mangled metal and debris strewn everywhere. The train's six coaches and two diesel tractors had all derailed, with several lying on their side.
The accident is the latest in a string of rail accidents in Britain in recent years.
In February 2001, a Land Rover crashed through barriers and ended up on the main east coast line at Selby.
A high-speed train then hit the vehicle, derailed and slammed into an oncoming freight train, leaving 10 people dead.
In May 2002, seven people died when the rear coach of a train heading from London to King's Lynn in Norfolk derailed.