The matter was raised during questions to the Deputy First Minister and Transport Minister Ieuan Wyn Jones this week following problems with capacity on rugby international days.
Leanne said: “There has been much talk of improving the line between London and Swansea by electrification, which is to be applauded, but there is also a need to improve the services that connect our capital city to the Valleys.
“The many thousands of commuters who use that service every day complain of cramped conditions because of the lack of carriages. The shortage of seats is felt particularly at this time of year, when the 6 Nations tournament is held, with supply frequently outstripping demand.
“Loyal rugby supporters are denied access to packed carriages on their journey into Cardiff and are then forced to wait for hours in long queues snaking around Cardiff Central railway station on the way home.
“What can be done to ensure that people, whether they are rugby supporters or just on their way home after a day’s work, can travel in comfort and receive value for money on the Valley Lines?”
Mr Jones, the Plaid Cymru leader, said he had held recent meetings with Arriva Trains about them meeting demand, particularly on rugby international days, and he said he would continue discussing the best way to tackle the issue.
He added: “The reality is that it (Arriva Trains) is struggling to provide the carriages necessary for those kinds of numbers, but I have agreed to work with them to see what we can do to assist and perhaps to provide alternative modes of transport.
“Perhaps First Great Western and some bus and coach companies could step in to relieve some of the pressure.”