There is anger that a multi-billion pound project to build a railway line between Oxford and Cambridge has been classed as an England and Wales project.
The £6.6bn line will see no tracks laid in Wales but, because of the way it has been classified, the country will not benefit from any extra cash.
David Chadwick, Liberal Democrat MP for Brecon, Radnor and Cwm Tawe, told BBC Radio Wales Breakfast it was "HS2 all over again", while former Welsh Labour transport minister Lee Waters called for Westminster colleagues to "fix" the system.
The UK government said the project is being financed through its "rail network enhancements pipeline", which it said was also funding schemes in Wales.
Under pressure over the issue in the Senedd, Wales' First Minister Eluned Morgan defended the decision but said Wales was not getting its "fair share" of rail cash.
Meanwhile the Welsh government's Transport Secretary said a "pipeline" of work had been agreed with the UK government.
The Tories said the Welsh government had "no sway" with its colleagues in Westminster, while Plaid Cymru called the decision "absurd".
Chadwick said Wales was "being denied hundreds of millions in funding that could transform our own rail network".
He added that Labour expected people in Wales "to believe the ridiculous idea that this project will benefit them and they are justified in not giving Wales the money it needs to improve our own public transport systems".
