A signalling problem between Didcot Parkway and Bristol Parkway means services between Swindon and Bristol Parkway may be cancelled or delayed at short notice.
An estimate for the resumption of normal services will be provided as soon as the problem has been fully assessed.
Simon Calder, a travel journalist, says the disruption at London Paddington train station could continue into the weekend.
A number of services, including trains to and from south Wales, were cancelled after signalling problems caused major disruption to a line.
The CEO of Real Crisps, owned by the Tayto Group, has confirmed a further 10 jobs will go at the end of the year.
90 people were made redundant when there was a fire at the Sirhowy factory in September.
Just under 80 of those people left at the end of October, with the others travelling to other sites.
Paul Allen says that those people no longer want to continue travelling so far from home and so will be leaving the company at the end of the year.
In total, 120 people were employed at the south Wales site with the remainder of employees working for the company at other sites.
According to Mr Allen there is a possibility the factory could re-open again but he says it is a "long and complicated process."
– Paul Allen, CEO of Tayto Group
We were very grateful that no one was hurt in this fire, but it is really devastating for the people who have been made redundant, especially coming up to Christmas.
We have tried to keep on as many of our employees as possible, some are travelling across to do shifts at our premises in Northern Ireland and others are still working locally. We have taken offices for six months near to the site of the old factory and we are trying to keep as many of the team together as we can. We have guaranteed work for all of these remaining employees, for the foreseeable future.
Grand Slam champions, Jamie Roberts and Lloyd Williams will be in Pontypridd later sampling an range of local produce on the last day of The Big Welsh Bite.
The food festival is the largest of its kind in south Wales and takes place between 11am and 5pm over three days.
Since the festival started on Friday it has attracted Welsh rugby stars Jonathan Davies, Rhys Priestland, Leigh Halfpenny and Alex Cuthbert.
Along with food there is also cookery demonstrations, farm animals, children's rides and a health and wellbeing zone.
In Pontypridd's town centre later the ever-popular Harnessed Horse Parade, a four-legged spectacular, will journey through the streets as part of the festival.Last year 17,000 people flocked to the event in Ynysangharad War Memorial Park.