Cracks on high speed trains made in the North East discovered in May 2021 were caused by several factors, an investigation by the Office of Rail and Road (ORR) has found.
Fatigue cracking on Hitachi's Class 800 trains resulted from "trains experiencing greater loads from train movement than allowed for in the original design", the ORR said.
Potential reasons for this are "wheel wear and track design".
The trains were made at the Hitachi factory in Newton Aycliffe.
Cracks were also caused by the use of a particular type of aluminium which was corroded by salt in the air.
The withdrawal of Class 800 trains until additional safety checks were introduced led to a week of severe disruption to services.
ORR’s HM Chief Inspector of Railways Ian Prosser CBE said: "With our oversight, Hitachi Rail and operators have put in place robust plans to make sure the right safety issues are being managed following the initial discovery of cracks on the trains, which have allowed trains to re-enter service.
"Safety remains the number one priority. Our review provides a clearer picture of the issue and we will continue our oversight to ensure work moves forward to agree the permanent solution and our recommendations are acted on.
"It is important that the whole industry works together to learn lessons from what has happened and our recommendations will help with that."
Affected operators were Great Western Railway, London North Eastern Railway, TransPennine Express and Hull Trains.