Transport for Wales (TfW) is awaiting permission from the Department for Transport to continue using outdated Pacer trains next year.
The operator is seeking a dispensation against accessibility requirements for trains which come into force on January 1.
Almost all Pacer trains will fail to meet the standards, which include provision for wheelchair users.
A spokesman for TfW said its 30 Pacers will “gradually be removed from service” as more modern Class 769 trains become available in the new year.
The firm had previously pledged to phase out the old trains by the end of 2019, but they are being kept “to improve capacity and resilience”.
Politicians have demanded that train operator Northern compensates passengers who have to keep travelling on Pacer trains after it admitted it would not adhere to its own promise to stop running them by the end of the year.
Greater Manchester mayor Andy Burnham, Sheffield City Region mayor Dan Jarvis and Leeds City Council leader Judith Blake have expressed to the company their “deep disappointment and frustration” at the situation.
The letter signed by the three leaders and sent to Northern managing director David Brown said the firm should reduce fares on routes where Pacers are being used until their replacements are brought into service.
Mr Jarvis said the current arrangement is “another example of rail passengers in the North being treated like second-class citizens”.
“It’s unacceptable that people will have to continue to travel on these relics, which should have been consigned to a transport museum long ago.
“A reduction in fares on affected routes, throughout the period that passengers have to travel on what are essentially buses on rails, is the very least that could be done.”
Pacers were built using bus parts and were introduced in the early 1980s as a stop-gap solution to a lack of rolling stock.
But their use has continued and Northern has said the trains will still be used in the Sheffield City Region and some other routes into 2020.
The politicians said the continued use of Pacers “underlines the widening disparity between transport investment in the North, compared to the South”.
A Northern spokesman said: “As a result of further delays in the construction and delivery of our new trains from manufacturer CAF, a small number of Pacers units will need to be retained for a short period of time in 2020 to deliver the planned daily timetable with the right capacity for our customers. This situation is not unique to Northern.
“We understand that customers will be disappointed and we are finalising proposals for customer support and offers for customers on those routes on which Pacers will be used in 2020.”
GWR is the only other operator still using Pacers.
It has eight in its fleet, with six in daily use on the line between Exmouth and Barnstaple in Devon.
A spokesman said fleet details from December are “yet to be confirmed” but all services will be wheelchair accessible with an accessible toilet.
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