More services on Great Western Railway are running, but passengers warned of fewer carriages and the possibility of having to change trains to get to some destinations including Oxford, Hungerford and Reading.
Trains with cracks which caused days of cancellations and delays for rail passengers, re-entered service after the rail industry assessed the fault does not pose a safety risk.
Services on large parts of the network have been disrupted since last Saturday after cracks were discovered in part of the chassis of several trains.
Mark Hopwood, GWR Managing Director, said: "Our customers have shown great patience over the past week, 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 over the weekend and reintroduce more consistent robust timetables for customers from Monday."
Manufacturer Hitachi, train operators and the Government have agreed a plan to reintroduce more Class 800 and 385 trains. Industry body the Rail Delivery Group said engineers and independent experts have "completed rigorous tests and research" to get a "clearer understanding of the cracking issue".
An industry source told the PA news agency that the investigation concluded that the cracks on lifting points - which allow carriages to be raised during maintenance work - do not stop trains from operating safely.
The RDG said trains will only re-enter service following "thorough inspections by specialist teams".
The announcement means Great Western Railway will be able to ramp up services. GWR said the announcement means it can run "some additional services" on Thursday and introduce "more consistent robust timetables" after the weekend.
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. The industry has come together to help support those travelling, with other operators allowing each other’s tickets to be used on their networks; adding in extra shuttle services to help move people; and in sharing rolling stock to provide it to those who need it most.”
Passengers are being advised to check the latest travel advice from their train operator. Eligible passengers are also being encouraged to claim refunds.