New trains urged for electrified Swansea to London line
Wales must not receive "hand-me-down" trains from around the UK when the railway between Swansea and London is electrified, Plaid Cymru says.
Party leader Leanne Wood is urging the Welsh government to lobby ministers in Westminster, who are leading the project, for new rolling stock.
The £350m investment will also see lines from Cardiff to the south Wales valleys electrified.
The Welsh government said it would try to ensure the best deal for Wales.
The Department for Transport (DfT) said the Welsh government would consider what rolling stock was required for the Valley lines.
The south Wales rail electrification, and the prospect of longer, faster trains, was announced by the DfT in July 2012.
Network Rail has said modernising one of the busiest lines into south Wales will cut 20 minutes off the three-hour journey between Swansea and London.
Passengers are expected to benefit from the new services by 2018.
But Ms Wood, the AM for South Wales Central, said Welsh rail passengers should not have to travel in rolling stock cast-offs from other parts of the UK network.
"I welcome electrification but I want to ensure that the valleys network does not receive a second class service when it is introduced sometime between 2018 and the end of 2019.
"Passengers in the Rhondda, where I live, already feel as though they get the rough end of rail services.
"Pacer trains, which are effectively buses on tracks, are not the most comfortable ride, they are around 35 years old and have no modern facilities on trains people see in other countries. There are also major capacity issues."
She said in the past old trains from Scotland and northern England had been brought in to serve the valleys.
"The decision will lie with the UK government but I would expect the Welsh government to make it absolutely clear that we should have new trains. This is a major test for them in terms of standing up for Wales," Ms Wood added.
The Welsh government spokesperson said: "We will do everything we can to ensure the best deal for Wales."
A DfT spokesman the UK government was committed to transforming the rail network in Wales, including a major programme of electrification for the Great Western main line and Valley lines, and the introduction of new state-of-the-art trains as part of the government's £4.9bn intercity express programme.
"We will continue to work closely with the Welsh government as it considers what rolling stock is required for these local Valley Lines services," he added.