The derailment of a freight train that caused a major fire happened because of a fault with its wheels and brakes, investigators have found.
The incident, which happened on August 26, saw ten of the train's wagons come off the tracks and spill 330,000 litres of fuel in Llangennech.
Police officers declared a major incident and 300 people were evacuated from their homes.
The Rail Accident Investigation Branch (RAIB) said a preliminary examination found that all the train’s wheels were "probably rotating freely".
The train had left Robeston oil refinery in Milford Haven, Pembrokeshire and was heading towards a fuel distribution terminal in Theale, Berkshire.
Investigators found the brakes on the third wagon became applied at some point during the journey, creating a flat spot on some of the wheels.
This caused the distortion of a section of track that then led to the derailment of the wagon, followed by another nine.
The driver “looked back and saw that a fire had started in the wreckage”, the RAIB said.
He uncoupled the locomotive from the first wagon and drove it around 400 metres away.
The driver was unhurt and reported the accident to a railway signaller.
Two workers escaped unhurt from the incident but a number of homes were evacuated, with around 300 people affected.
Speaking at the time of the blaze, Dyfed-Powys Police Superintendent Ross Evans thanked those living in the area for their co-operation.
"This was a fast-moving situation, which was declared a major incident due to the initial risk of harm to people living in the area," he said.
“The evacuation no doubt caused a significant amount of disruption for those evacuated in the middle of the night, and I would like to personally thank residents for their response, as well as our frontline officers who swiftly moved hundreds of residents out of their homes."
There were fears that diesel spillage into the nearby River Loughor could spell disaster for the local cockle fishery industry, with the Food Standards Agency advising the closure of shellfish beds in the area’s estuary.
The RAIB said its investigation will consider why the brakes on the third wagon became applied, the maintenance history of the wagon, whether any other factors contributed to the derailment and how it led to the fuel spillage and fire.