Flooding caused by Storm Brendan is affecting trains running between Cardiff and London Paddington.
An area of line between Bristol Parkway and Swindon was badly affected by the storm that hit parts of the UK earlier this week.
National Rail says that all lines are now open but some train services many still be cancelled.
On Thursday morning they said: "This is expected until the end of the day."
When the line was closed services between south Wales and London were diverted via Bath Spa.
Speed restrictions were also put in place.
The advice from National Rail is to "check before you travel".
