The derailment of a diesel-carrying freight train which led to a major fire happened after some of its wheels were damaged by a fault with the brakes, investigators said.
Ten of the wagons came off the tracks and spilled 330,000 litres of fuel as the train passed through Llangennech, near Llanelli, South Wales on August 26.
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” when it left Robeston oil refinery in Milford Haven, Pembrokeshire, but “at some point during the journey” the brakes on the third wagon became applied.
This created a flat spot on some of the wheels, leading to a section of track being distorted and the subsequent derailment of the third wagon, followed by another nine.
The locomotive and the first two wagons came to a stop about 180 metres away.
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.
The train was travelling at around 30mph when the incident happened.
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.
The train, owned by DB Cargo UK, was travelling to a fuel distribution terminal in Theale, Berkshire.