East Somerset Junction derailment 'caused by track fault'
A freight train derailed because of a design fault in a section of replacement track, a report found.
The derailment, between Castle Cary and Frome on 20 March, caused major travel delays and took four days to repair.
A section of track had had a set of points removed and a curved plain-line section of track added, in 2010.
Investigators found the right-hand rail was "insufficiently restrained, allowing it to rotate outwards and become unclipped from the base plates".
The inquiry identified the design of the track was "sub-optimal" and signs of gauge spread [rails moving away from each other] were not identified during inspections of the track by staff from Westbury track maintenance depot.
Among the formal recommendations of the Rail Accident Investigation Board (RAIB) report, it said: "Network Rail should enhance its procedures... [and] appropriate measures should be included to manage the risk where the newly-installed plain line is curved.
"Consideration should be given to limiting the duration of such installations without an independent inspection, permanent design and/or track renewal taking place".
It also recommended that there should be "adequate resources" brought in to allow track maintenance in the Westbury area.
The RAIB also made a learning point, reinforcing the importance of identifying "gauge-spread" on sections of curved track that may be subject to high lateral loads.
A learning point is an issue the RAIB uses to reinforce safety procedures which are not covered by a formal recommendation.
RAIB investigations are carried out in order to improve rail safety to help prevent future accidents or mitigate their consequences, as opposed to establishing blame or liability.