The regulator also commends the rapid industry response and subsequent work to safely reintroduce trains to service.
Cracks found in the area of the bolster close to the yaw damper bracket and anti-roll bar fixing points were a result of fatigue, and cracks found in the area where the lifting plates attach to the vehicle body were the result of stress corrosion cracking.
Stress corrosion cracking occurs when susceptible materials are exposed to a specific corrosive substance while subject to stress.
ORR’s report found that once the cracks were identified, the industry worked collaboratively to swiftly withdraw the trains and then to establish a suitable process for assessing which trains were able to return to service.
The regulator’s interim report finds that since being put back into service, trains have performed as specified, with no unsafe conditions or harm arising from the cracking.
ORR continues to work with Hitachi and the industry and expects to publish a final report in December 2021.
HM Chief Inspector of Railways at ORR Ian Prosser CBE said: “Our interim findings confirm the cracking in the yaw damper and lifting plate are a result of fatigue and stress corrosion cracking – and that Hitachi made the sensible decision to withdraw all trains. Since then, the majority of trains have been put back into service with no unsafe conditions and no harm arising from the cracking. I welcome the good collaboration that has taken place since this issue arose. We are continuing to work with all parties to determine the root cause and will publish our final report in December.”
ORR’s final report will establish the root cause of the cracking and will review Hitachi’s plan for long term fleet recovery and management, and identify any areas for improvement for the industry.