Delays and cancellations are continuing across Transport for Wales services after a shortage of staff caused disruption over the weekend.
It comes as new timetables and extra services were introduced on Sunday in what the network described as the 'most significant changes to train times on our network in the last ten years'.
A spokesperson for the service said on Saturday: "We are continuing to do everything possible to ensure the maximum number of services are operating.
"This time of year can always be challenging, particularly on weekends and we’re sorry that there have been a number of cancellations on our network today."
Passengers have taken to social media to report disruption across the network.
The changes in the rail network are part of a major timetable change to Britain's railway services, with more than 1,000 new train services being introduced across the UK.
Changes to Transport for Wales services will include new Sunday routes, increased frequency of services with additional earlier and later departure times.
The launch also marks the rollout of newer and refurbished trains.
Why are there so many timetable changes?
The main factor affecting services that run along the South Wales mainline is the electrification of the line between South Wales and London.
Among the changes include the first non-stop trains between London and Bristol in decades.
Which routes will be most significantly affected?
Routes between Cardiff and Holyhead, Cardiff and Swansea and Maesteg and Cheltenham will be most affected by the changes.
Transport for Wales said the need to accommodate the retiming of the South Wales to London services and extra loco hauled trains have resulted in different calling patterns along the Cardiff to Holyhead route.
It also has a contractual requirement to offer faster services between North and South Wales.
Where are the extra Sunday services?
Transport for Wales is increasing its Sunday services by around 40 per cent due to higher demand.
The routes with extra Sunday services include:
Rhymney – Cardiff
Treherbert – Cardiff
Barry Island – Cardiff
Cardiff Bay Shuttle
Maesteg – Cardiff
Cardiff Central – Swansea
Shrewsbury – Crewe
Pwllheli – Machynlleth
Aberystwyth – Shrewsbury
Chester – Crewe
Llandudno Junction – Llandudno Town
Llandudno Junction – Blaenau Ffestiniog
Holyhead to Manchester
Which routes will new trains run on?
Transport for Wales will also introduce newer and refurbished trains on certain routes over the next six months.
It said not all routes will see newer trains as "some trains are more suitable for moving lots of commuters, with short distances between stations, so a very fast train may not be necessary."
The routes getting newer and refurbished trains include:
Maesteg – Cheltenham
Ebbw Vale – Bridgend
Holyhead – Manchester
Wrexham – Bidston
Holyhead – Cardiff
Will this mean the trains are less overcrowded?
In October, ITV Wales heard from passengers who travel on the network regularly and said overcrowding, delays to services and poor conditions were a common occurrence.
Bethan Jelfs, Customer Delivery Director at Transport for Wales, said the new trains will have extra capacity.
"This timetable does see the introduction of some new rolling stock.
"We are therefore able to offer more capacity on our peak trains, particularly on our Valley lines.
"We're hoping that our services and our reliability will start to improve."
Why has it taken so long for these improvements to be made?
Ms Jelfs said: "We always knew this was a long journey for us, to get the trains. We can't just buy trains as you can a car, so it takes time.
"Trains have been in transit to us, there have been some delays with those trains and the manufacture of them."