The First Great Western train, en route to London Paddington from Swansea, came off the tracks near West Ealing Station at 2005 GMT on Sunday.

There were no injuries and about 500 passengers were evacuated from the train.

Network Rail has described the problem as an "infrastructure failure at a rail joint".

No-one is reported to have been injured

It is understood investigators have found a broken fish plate, which connects the ends of two rails.

Engineers are not yet clear exactly how the broken plate contributed to a set of wheels leaving the track.

Network Rail said the there was no break in the rail itself, nor a points failure - the two faults which caused fatal crashes at Hatfield and Potters Bar.

BBC News Online's Andrew Walker, who was on the train, told of the moment it partly derailed.

"There was a great bump and a crash," he said.

"We were all thrown in the air. It felt like the train was being dragged slightly."

John Roche, a spokesman for Great Western Railways told BBC News Online it was fortunate no-one had been injured given that the train travels at 125mph along that track.

"The train was between two stations which has made it more of a problem than if it was in a station," he added.