A freight train which derailed and caught fire in South Wales consisted of 25 wagons each holding up to 100 tonnes of diesel or gas oil, investigators said.
The accident happened as the train passed through Llangennech, near Llanelli, at 11.15pm on Wednesday.
The Rail Accident Investigation Branch (RAIB) said the derailment and subsequent damage to the wagons “resulted in a significant spillage of fuel and a major fire”.
Local people reported “seeing a plume of flames and smoke, and the strong smell of fuel”, the RAIB added.
Ten of the wagons derailed.
The driver, who was unhurt, reported the accident to a railway signaller.