The power unit fell from the top of the train

The train was approaching Watford on Monday when the incident happened.

One of the passengers said they were left in the dark by staff who did not know what to do.

A spokesman for Virgin Trains apologised for the inconvenience and said an investigation has been launched with Network Rail.

'Two loud bangs'

Tom Poole, who was on the train, told BBC News Online: "The whole thing was a fiasco.

"I was sitting directly underneath the power unit when it fell and I heard two loud bangs which was alarming. Another passenger saw lots of sparks.

"I went to the restaurant car and asked what was being done but they didn't seem to be worried."

He added passengers were left on the train for a further 15 minutes before eventually being told to get off.

Mr Poole said worse was to follow when a tannoy announcement said a train approaching the station would stop to pick up passengers - but carried on straight through.

The final insult, he said, was when a replacement service took passengers to Birmingham International and they had to get another train to New Street - arriving four hours after the start of the journey, twice what it should have taken.