A photo taken by a passenger after the level crossing collision
The car was dragged for hundreds of yards after being struck by the 1.36pm Greater Western London to Cheltenham Spa service.
One passenger said he heard a "massive thud" before the train came to a "sudden stop".
Another said a Land Rover had been hit, with photographs posted to social media showing a crumpled vehicle at the front of the train.
Emergency services rushed to the scene but sadly the man was pronounced dead at the scene.
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Investigators said: "At present, officers are currently examining the scene to establish the circumstances which led to the man’s car being struck.
"Anyone with any information is asked to contact BTP by calling 0800 40 50 40 quoting reference 311 of 07/02. Or you can send a text to BTP on 61016."
The crumpled car after rail staff covered it with a tarpaulin
Rail staff at the scene of the accident
A spokesman for South Western Ambulance Service said the victim was a 60-year-old-man.
He said: “The call was received at 3.03pm to Frampton Mansell to reports of a two-vehicle collision. There is one male casualty, who is 60-years-old.
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Emergency services following today's tragic collision
“We are in attendance with two ambulances and a rapid response car.”
Rail lines between Swindon and Gloucestershire are now shut.
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