Engineering work is taking place between Bridgend and Baglan, closing some lines.
The 07:55 Swansea to London Paddington service will start from Cardiff Central at 09:00.
A rail replacement bus service will run between Swansea and Cardiff Central, departing earlier at 07:50. Another bus will run between Neath, Port Talbot Parkway, Bridgend and Cardiff Central.
Check before you travel:
You can plan your journey using the National Rail Enquiries Journey Planner.
Replacement bus travel advice:
For helpful advice if you need to travel on a rail replacement service, including accessibility and bicycle information, please use our Rail Replacement Services page.
You can find the pick-up and drop-off location of rail replacement services by checking station signage or by searching for your station on our Find a Station page.
Please be advised that replacement vehicles may be busier than usual, and you should allow extra time for your journey.