The Pacers were bought in the 1980s, when money was so tight British Rail couldn't afford to buy proper trains.

The idea was to use bus technology to keep costs to a minimum. But while the trains may have been cheap to buy, since then they have been a never-ending drain on finances.

At the same time, they could have damaged the railway's revenue, deterring potential passengers because they are cramped, uncomfortable and unreliable.

The trains have been modified, at significant cost, to reduce breakdowns. But the ones used in South Wales are the most unreliable trains in Britain, and are becoming even worse despite Arriva's protestations that it is improving them.

Similar and even identical trains in England are faring better, including the ones maintained by Arriva in Cardiff for another company.

The inescapable conclusion is that the Valley Lines are too tough an environment for these trains. One obvious difficulty is the steep hills they have to climb, day in day out, many times a day.

Equally problematic is the short distance between stations. There are 10 stops on the Rhondda line after Pontypridd, where there might be just two or three in the same distance on other rail networks. That means the powered doors have to work overtime, and a door failure can cause a service to be delayed or cancelled.

It beggars belief that Arriva has no plans to replace these trains during a franchise that takes us through to 2019. The company was never-theless happy to increase its fares yesterday by more than inflation.

It is time that we - the passengers and taxpayers who fund Arriva's franchise - stopped throwing money at these substandard trains. There is something seriously wrong somewhere if a commuter railway which is witnessing so much growth in demand cannot afford to provide adequate rolling stock.

Welsh Liberal Democrats believe the problem lies in the system of leasing trains from private companies. If that is true, Arriva and other interested parties, especially the Welsh Assembly Government, must find an alternative route.

New trains would bring other benefits besides better reliability. They would provide more capacity for passengers and be more comfortable. They would also meet the latest disability regulations, making travel easier for parents with prams, passengers with heavy luggage or shopping, people with hearing or visual impairments and others.

The new trains should be ordered in greater quantity than the old ones they will replace. This is to cater for the existing trend of growth in passengers, and for the additional passengers who would be attracted to the railways by the new trains' superior reliability, capacity, comfort, accessibility and image.

It's time to put the Pacer trains - and Welsh passengers - out of their misery.