Passengers using trains from west Wales to places like Manchester have been complaining of overcrowding.
It comes as Transport for Wales confirmed that there is a current shortage of trains across the Welsh network.
In a video message posted on the rail operator's social media accounts, the cause of shortage was explained.
Jan Chaudhry-Van der Velde, Chief Operations Officer at TfW, said that a number of class 175 trains had been pulled from passenger service due to safety concerns.
They are currently receiving "enhanced maintenance" but he added that as the trains "go through the programme, they'll return to passenger service".
He asked passengers to "bear with" staff as worked is carried out.
But the current shortage has led to some passengers sharing their frustrations on social media, including Jack.
On Twitter, he said: "If you value your legs, have difficulty balancing on rickety trains or don’t enjoy the breathe of many people blowing constantly in your face then avoid this rail service at all costs!"
In response, Transport for Wales apologised and said that they know capacity is one of their "biggest challenges" and staff "continue to explore all options to improve capacity" for customers.
Meanwhile, Transport for Wales is bringing in new trains to run across the Welsh network.
Passengers in north Wales have started to see the class 197 trains run on the Conwy Valley Line between Llandudno and Blaenau Ffestiniog.
In south Wales, the first of the class 231 trains is now operating on the Rhymney line with more being introduced throughout the year.
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