The immediate cause of the derailment was the flange of the leading right-hand wheel of the leading bogie of wagon 610117 climbing over the right-hand rail at the point where it changed section from UIC60 to BS113A.

The causal factors were: the track had been installed with a radius that was sharper than designed and was not fitted with a check rail; one of the transition rails had been installed incorrectly; the frame of the vehicle which derailed was twisted, and the sidebearers of this wagon were fitted with packings that did not compensate for this twist.

The RAIB has made four recommendations concerning: a system to assess and mitigate, so far as is reasonably practicable, the risk arising from twisted frames on container wagons and audit compliance with it; checking other sites where the same project team had installed track to verify that it has been designed and installed correctly; updating Network Rail’s processes for track management to include checks that the rail fastening components are of the correct type for the particular rail and sleeper combination; and updating Network Rail’s track recording information handling process to deal with gauge variation and should issue guidance to staff on minimum actions to be taken at each alarm level.