Train passengers were left "distressed" and "confused" when the driver of a packed train encouraged them to get off at a station for some fresh air – only to leave without them.

Trevor Greenfield, 74, and his wife, Eileen, 68, were among the group of eight passengers stranded at Swindon, having boarded the train from London Paddington to Carmarthen. "People were distressed, confused and flabbergasted," said Mr Greenfield, from Barry in Vale of Glamorgan. "It's worrying and disappointing that the driver made no announcement that the train was leaving."

Great Western Railways apologised for the inconvenience caused.

Mr and Mrs Greenfield boarded the train from London to south Wales on 10 April, but found it packed with passengers because two earlier trains had been cancelled. They had to stand with 10 others in hot and cramped conditions for about an hour until the train stopped at Swindon and was delayed for about 15 minutes.

Mr Greenfield said the driver announced he would arrange for the train guard to open the door so they could get off and get some fresh air during the wait. He said a group of eight people got off and stood about a metre from the train, waiting to be told when to get back on.

But at about 17:30 BST, with no announcement made, the doors of the train closed and it started moving. "As a group, we were horrified and in shock that it was pulling away. People started shouting and waving in panic to the guard who was some distance away," Mr Greenfield said.

The couple had also left their suitcase – containing clothes, medication and valuables for a four-day break in London – on the train. They were eventually able to get a train from Swindon to Cardiff, where their daughter drove from Barry to pick them up, and they were reunited with their luggage.

Mr Greenfield has lodged a formal complaint with GWR, describing it as a "long, stressful and worrying day".

Another passenger, Liam Svensen, 23, said the incident had been "disappointing and unacceptable. There was no announcement whatsoever to tell us to get back on the train. It was very out of order," he said.  He added that had his work colleague not got back on to the train before it pulled away, he could have lost £3,500 worth of work tools.

A spokesperson for GWR said: "We'd like to apologise to customers affected by the disruption last Thursday afternoon. To help keep the journey moving, the 16:18 Paddington to Carmarthen train made an unscheduled stop at Swindon to allow a relief driver to board. During the stop, the doors were opened. Our station and on board teams carried out the necessary checks before the train was safely dispatched. We understand that some customers did not re-board the train during this time, and we're really sorry for the inconvenience this caused."