A freight train which crashed on route to Theale causing a huge fire and threatening an environmental disaster was derailed by a brake defect, an investigation has found.

Deficiencies in the design and maintenance of brake components caused 10 of the diesel-carrying tanker train’s wagons to come off the tracks at Llangennech near Llanelli, South Wales, on August 26 2020, the Rail Accident Investigation Branch (RAIB) said.

Around 300 people were evacuated due to the extent of the blaze, which took firefighters nearly two days to put out.

Around 116,000 litres of diesel were burnt, with a further 330,000 litres spilling into the local area, causing major concern for waterways and wildlife.

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The railway line was closed for more than six months.

The fault caused a set of wheels on the third wagon to stop rotating during the journey from Robeston oil refinery in Milford Haven, Pembrokeshire, the RAIB’s report said.

The wheels were unable to safely negotiate a junction, leading to the derailment.

Natural Resources Wales compared the incident to the Sea Empress disaster of 1996, when a tanker spilled 72,000 tonnes of crude oil and hundreds of tonnes of fuel after running aground off Pembrokeshire.

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The Welsh Government-sponsored body led a major environmental recovery operation, which included excavating 30,000 tonnes of fuel-soaked soil.

It continues to monitor the area to ensure the safety of harvested cockles and shellfish.

Chief inspector of rail accidents, Simon French, said the consequences of the incident were “disastrous” and will “take years to put right”.

He acknowledged trains carrying dangerous goods “play an important role in the UK economy” but said the risks “must be adequately controlled”.

Problems with the maintenance of freight trains have been identified by the RAIB in more than 10 accidents over the last decade.

The rail industry’s approach to the issue “needs to improve”, Mr French said.

He added: “I would like to stress the importance of getting this right.

“It’s time that freight wagon maintenance practices were subject to careful examination and for the industry to think through the way that it can best deliver on its legal and moral obligation to present wagons that are fit to operate through the nation’s towns and cities.”

The train, owned by DB Cargo UK, was travelling to a fuel distribution terminal in Theale, Berkshire when the derailment happened.

The RAIB made a series of safety recommendations, including that the manufacturer of the faulty components reviews its design.