At about 23:15 hrs on 26 August 2020, train 6A11, the 21:52 hrs freight service from Robeston (Milford Haven) to Theale, conveying 25 tank wagons, each containing up to 75.5 tonnes of diesel or gas oil, derailed on the ‘Up District’ line near Llangennech, in Carmarthenshire. The derailment and the subsequent damage to the wagons resulted in a significant spillage of fuel and a major fire. Subsequent examination of the site revealed that a total of 10 wagons (positioned 3rd to 12th in the train) had derailed, and that around 330,000 litres of fuel had been spilt.
RAIB’s preliminary examination found that, although all the wheels of the train were probably rotating freely when the train left Robeston, at some point during the journey the brakes on all wheels of the third wagon of the train had become applied, and remained so until the derailment. Although their brakes were dragging, three of the four axles of this wagon continued to turn. At some point in the journey the leading axle ceased to rotate altogether, as evidenced by a CCTV camera located at Pen-y-Bedd level crossing (12 miles (19 km) from Morlais Junction), and marks on the railhead.
The locking of the leading axle of the third wagon caused the development of a flat spot around 230 mm long on both of the wheels on this axle. This in turn created substantial ‘false flanges’ (raised lips on the outer side of the wheel treads). When the train reached the crossover at Morlais Junction, travelling at about 30 mph (48 km/h), the false flange on the right-hand wheel caught on the converging stock rail and distorted the track, leading to derailment of both wheels. Around 100 metres further on, the partly derailed wagon encountered facing points set to route the train to the right. The locomotive and the two leading wagons went to the right and the derailed third wagon went straight ahead. The third wagon turned over onto its right-hand side and became detached from the wagon in front of it. This caused the points and the track beyond them to be destroyed, and derailment of another nine wagons followed.
Examination of the brake group on the third wagon (TEA wagon GERS89005) found that the relay valve was loose on the pipe bracket. A sealing ring from one of the ports in the mating face had migrated to a position where it lodged between two other ports, distorted the sealing rings of those ports, and probably created a route for air to pass directly from the auxiliary reservoir to the wagon brake cylinder. This would have had the effect of applying the brakes on the wagon, producing the result described above.
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