Design work has begun on a £113m upgrade to Wales' busiest station.
UK Transport Secretary Grant Shapps has released £5.8m to Transport for Wales to improve access and capacity at Cardiff Central.
There is also money for signalling on the Cambrian line, and for exploring ways to speed up trains between Llandudno Junction and Chester.
The Welsh Government called for more funding to address under-investment and catch up with spending in England.
The cash for Cardiff Central is part of the £58m announced for the project last year.
Mr Shapps also announced £3m to "advance plans" for the upgraded signalling on the Cambrian line, intended to be completed by 2023.
Almost £2m has also been made available for the "next phase of development work" on proposals to speed up journeys in north and south Wales.
The schemes - exploring better aligned tracks and additional services on the lines - cover routes between Cardiff and Swansea, Chester and Llandudno Junction, and the Severn Tunnel and Cardiff.
The UK government said it was making "major progress" on upgrading Welsh railways, listing recent investments worth £343m.
The sum includes previously announced projects such as the Cardiff Central upgrade, £3.9m for Bow Street station in Ceredigion due for completion in 2021, £2.9m for almost finished accessibility improvements at Cadoxton in the Vale of Glamorgan and £76m to electrify the Severn Tunnel.
The latter project, completed in June, means electric trains can now run from Cardiff to London.
The UK government is also providing £196m as part of the transfer of the core Valleys lines network to Transport for Wales, which was completed in March and which will form the basis of the Metro in south Wales.
The figure is larger than the £125m previously thought to be the UK government's share of the scheme.
Mr Shapps said: "The host of improvements we are delivering across Wales, from huge infrastructure upgrades to creating new and modern stations, are vital to deliver better, quicker and more convenient journeys for passengers within, into and out of Wales."
The existing Cardiff Central station, a Grade II listed building with a Portland stone booking hall and art deco lighting, was built in the early 1930s.
Between April 2018 and March 2019 it was by some margin the busiest station in Wales - with 14.2 million entries and exits, compared to 3.4 million at the second busiest station, Cardiff Queen Street.
But public transport has been hit by the restrictions surrounding the coronavirus pandemic, and for much of the last few months was restricted to essential journeys.
That restriction was lifted in Wales on 17 August.
The £113m Cardiff Central upgrade scheme is expected to be funded by £40m from the Cardiff City Region group of local authorities and £15m from Transport for Wales Rail Services, as well as the cash from the UK Department for Transport.
Work is hoped to start in 2022, although the design and development exercise will determine how long the scheme will take.
A Welsh Government spokesman said: "This funding is a step in the right direction but much more is needed to catch up with rail infrastructure spending in England.
"Rail infrastructure is the responsibility of the UK government and we have been clear about the urgent need to make up for decades of under-investment in Wales. That includes the cancellation of key projects, such as rail electrification between Cardiff and Swansea."