Rail bosses have been given the green light to keep using older trains which do not meet new accessibility rules coming into force on New Year's Day.
Transport for Wales services are among those affected, with firms having had 10 years warning of the need to comply.
UK transport minister Chris Heaton-Harris said he was giving the extension as long as operators proved new or refurbished trains were still on track.
Extra services and capacity on trains in Wales were introduced this month.
New Porterbrook-owned Class 769 trains, which meet disability laws, were due to have been delivered to Transport for Wales (TfW) last spring.
Opposition parties in Wales have accused the Welsh Government of not acting more quickly to tackle capacity problems on the rail network stretching back many years.
Plaid Cymru claimed the Valley lines were set to lose half of their rolling stock, and warned that other parts of Wales would have seen services cut to plug the gap.
In a letter to rail bosses across the UK last week, Mr Heaton-Harris said it was "extremely disappointing" that operators would fail to meet the deadline for accessible trains.
But he recognised there would be a "disproportionately negative effect" on passengers if all non-compliant carriages - around 1,200 of them - were taken out of service on New Year's Day.
It has been granted permission to keep using the older trains - including 33 Pacers - until the middle of 2020.
TfW chief executive James Price said: "Now that we have the necessary permissions from government, our plan is to retain some of our Pacer and Class 37 loco-hauled trains for a short period of time in 2020 to allow us to increase capacity for our busiest routes, meaning extra space onboard for our customers."