Great Western Railway has been given permission to begin reintroducing its Hitachi 800 trains.
Some were pulled from use across the country at the weekend after cracks were found.
But after passing certain safety checks, the trains are now permitted to return to service - which includes London Paddington to Penzance, Exeter St Davids, Bristol Temple Meads and Bristol Parkway.
Passengers are being advised to check the latest travel advice from GWR and find out if they are able to claim refunds.
GWR and Hitachi say engineers and independent experts have completed rigorous tests and research to gain a clearer understanding of the cracking issue.
They say trains will only re-enter service if they meet agreed safety criteria and a 'Forward Repair Plan' will ensure the long-term continued safe running of the fleet.
'Industry has come together'
Mark Hopwood, GWR Managing Director, said: “Our customers have shown great patience over the past couple of days, and I am grateful for their understanding as we have worked with Hitachi to allow trains to return safely.
"This news will allow us to run some additional services today and reintroduce more consistent robust timetables for customers after the weekend.
Andrew Barr, Group CEO of Hitachi Rail, added: “Today’s agreement sets out our joint plan for the phased reintroduction of our trains into service, which will continue to deliver the highest possible safety standards.
"Safety remains our number one priority, and we and our partners have worked round the clock to agree an approach that allows the return of trains to service where they have been deemed safe."