Space on trains for up to 6,500 more commuters a week on Valley lines has been announced by Transport for Wales (TfW) from December.
But the Welsh Government is to request a delay in introducing new regulations to make trains accessible for people with disabilities.
This is due to a backlog in train orders that has an impact throughout the UK.
TfW said Valley lines would see more four-carriage trains on peak services.
The Welsh Government-owned body is also introducing additional trains right across the Wales and Borders rail network, and carriages on some train services have been upgraded.
But Economy Minister Ken Skates said TfW would not be able to meet the January 2020 deadline for the new regulations to make trains accessible for people with disabilities without significantly cutting the number of trains operating.
He will ask the UK government for the organisation to be allowed to run some trains that do not comply with the new access rules “for a very short time”.
Mr Skates said he had faced a “stark choice” and one he made “reluctantly” and in common with other parts of the UK.
“While we have been working very hard to mitigate the impact of the wider supply issues on TfW’s service, this has left us with difficult decisions to make around TfW’s immediate compliance with passengers of restricted mobility (PRM) accessibility regulations,” he added.
Meanwhile, TfW said passengers between Cheltenham and Maesteg, and between Cardiff and Ebbw Vale would be travelling on “more modern trains with more space, accessible toilets, air conditioning and Wi-Fi and power sockets”.
Long-distance passengers on some services between north Wales and Manchester will be travelling in more modern carriages.
The announcement follows the release of TfW’s first annual report.
It said a “no-deal” Brexit could lead to increased costs and risk the supply of essential materials, including fuel, and restrictions on the free movement of people and goods could cause problems.
However it added it was taking steps to manage the potential impact on rail services.
It comes as the UK government was setting out its proposals for a Brexit deal.
TfW took over the operation of the Wales and Borders franchise from Arriva Trains Wales in October 2018.
The report set out TfW’s performance over the last year, its plans for the future and various risks it could face.
Control over the Core Valleys Lines upgrade is now being transferred to TfW from Network Rail in order to develop the £738m South Wales Metro.
But the report warned the construction industry had “a historically poor track record” of delivering complex rail projects “on time and on budget”.
Should this happen with the Valleys line project, TfW would either have to “seek further funding from the Welsh Government or reduce the delivery scope of the project”, it said.
It said it had plans in place to manage this process.
The annual report also said:
- £151m had been spent in its first financial year including £40m on its current fleet
- Sunday services increased by 61% by late 2019
- A new service between Wrexham, Chester and Liverpool had been introduced
- It had seen an improved gender balance with women making up 48% of senior management and 43% of staff, with a gender pay gap of 18.6% compared with the UK average being 17.9%
- Chief executive James Price, a former senior civil servant in the Welsh Government, is paid £155,000 to £160,000 a year.