Arriva Trains Wales have apologised to people using their Valley Lines rail links to Cardiff after increased complaints.
Commuters have said the trains are overcrowded, late and sometimes do not turn up at all.
Arriva Trains admit one in 10 trains in Wales was late last month.
Ian Trevallis, a businessman in Aberdare, said his train had been consistently delayed.
He said he was “concerned about the future and ensuring the valleys get a good deal.”
Another commuter, Laura, said: “the main problem is the old, dirty, cramped and late trains are the only direct option from Aberdare to Cardiff.”
Figures supplied by Arriva show that since the company took over the railways in 2003, the number of passengers regularly using the Valleys Lines had almost doubled to 31,000.
The company say this puts pressure on services, as the number of trains has remained the same.
James Nicholas, a spokesman for Arriva said that in partnership with the Welsh Government, they have spent £1m on purchasing five extra trains.
They will be introduced this year, but first have to be converted from electric to diesel.
He said Arriva were “doing the best we can with what we’ve got” but it was difficult to source and buy extra trains to cope with demand, because diesel trains are not made any more.
Mr Nicholas apologised for inconvenience to passengers, and said compensation was available “in extreme cases” for people that had been inconvenienced.
