Passengers are angry that many trains are now shorter than they used to be.

Services between Cardiff and Bristol, some of which continue to Bath and Portsmouth, are among those affected.

The RMT union called on Railways Minister Tom Harris to end the "scandal of increasingly hazardous" overcrowding by releasing into service mothballed trains.

RMT general secretary Bob Crow said, "It is nothing short of an act of deliberate sabotage of the region's rail services that long-suffering commuters are shoehorned daily into fewer and shorter trains - and railway staff bear the brunt of the anger and frustration experienced by rail users - while we now see serviceable trains withdrawn from service to be squirrelled away in railway sidings.

"As commuters return to work following the Christmas break they must be dreading the appalling mess that FGW have created."

He said there had been reports of passengers fainting from overcrowding and of them being left behind on platforms. More than 60 services have been cancelled each day, on average, since FGW introduced a new timetable on December 11.

"If the management responsible for the current monumental fiasco aren't up to the job, the Minister should take the Greater Western franchise in-house and run it directly in the public sector, where it belongs," said Mr Crow.

"FGW's ineptitude must not be used as an excuse for continuing to allow serviceable trains to languish in rail sidings and yards when the public needs them to provide train services essential for the region's environment and economy."

The official watchdog body, Passenger Focus, is also concerned about conditions passengers encounter on FGW trains.

Simon Pickering, Passenger Focus manager for Wales, said trains between Cardiff and Bristol were regularly overcrowded.

"It has led to people being unable to get on trains," he said.

FGW's maintenance problems follow its decision to move work from Arriva's Cardiff Canton train depot to Bristol.

A FGW spokesman said the company had more trains than contracted in its franchise agreement.

"Maintenance issues have led to shorter trains than usual. We are addressing these issues urgently," he said.

Two-hour delays for Mid Wales travellers

Rail passengers on Welsh lines are being delayed by up to two hours because trains are being turned around short of their destinations.

Arriva Trains Wales is resorting to the measure where running a train to its scheduled destination would delay the train's next journey and potentially other trains on the same line.

If the train completes more than half its journey, official records will not class it as cancelled when it turns around short of its planned terminus or fails to begin at its advertised starting point.

However, it will be recorded as a serious delay in ATW's performance statistics.

Rail watchdogs say trains are being turned around short of their destinations too often.

They say at times one in three trains on the Aberystwyth-Birmingham line has been terminated at Wolverhampton, where it starts the journey back for Aberystwyth.

Passengers going to Birmingham can catch the next train from Wolverhampton.

However, passengers travelling from Birmingham to stations in Mid Wales may not have enough time to catch an alternative train and reach Wolverhampton before the Aberystwyth service departs.

The next train to Newtown and other stations to Aberystwyth leaves two hours later.

Passengers in Pembrokeshire are also being delayed.