Six trains, part of a new high-speed fleet, have been taken out of service after hairline cracks were discovered.
Great Western Railway (GWR) said the cracks were found during routine maintenance of two of its Hitachi 800 trains.
It said "possible issues" had been discovered on another six trains, which had been withdrawn from service while investigation work was carried out.
Their withdrawal had no impact on passenger services.
GWR said the hairline cracks were "in areas where the suspension system attaches to the vehicle body on two trains".
Both have been scheduled for repair. Four of the other six being inspected have since been cleared to go back into service.
The fleet of Hitachi 800 intercity trains entered service in 2017 and was designed to be electric, but due to delays in electrification of the line engines were also be fitted with diesel power.
They are running on GWR's network between London, the Thames Valley, Bristol and South Wales.
The new trains were made at the Hitachi factory in Newton Aycliffe County Durham.