Ten of the wagons derailed spilling 446,000 litres of fuel, three of the tanks caught fire consuming about 116,000 litres. The remaining fuel spilled into an area of special scientific interest and a special area of conservation. Local residents were evacuated late at night by emergency services and it took 33 hours for the fire and rescue service to extinguish the fire.
The UK Rail Accident Investigation Branch (RAIB) investigation concluded that the derailment was likely to be due to the brakes being applied on one set of wheels during the journey. The sliding of the wheels on the rail caused a flat to be formed. The evidence indicates that the brakes had been applied due to a fault on a relay valve joint in which two M10 nuts had become loose, this allowed air to pass and activate the brake actuators. The wagon with the brakes applied derailed when passing a set of points when passing a set of points decoupling itself from the train and derailing several of the wagons behind it.
When a previous service was completed on the relay valve, nuts had been re-used and washers missed off the assembly. This caused a high friction condition that resulted in the clamp force being provided falling below that needed to ensure the joint’s structural integrity. This is assuming that the nuts had been tightened correctly to the specified tightening torque. This resulted in the nuts being loose and allowing a gap to open in the joint. This provided opportunity for one of the ‘O rings’ to migrate within the joint allowing air to pass into the brake actuator.
Originally, the brake relay valve was fitted to the mating surface of the pipe bracket by four M8 studs. A design change around 1985 reduced the number to two studs and increased the thread size to M10. This design has an inherently higher risk. If one of the two fasteners is not tightened, or not adequately tightened, the moment loading acting on the joint would tend to rotate the joint around the remaining tightened stud. Even if fully tightened, this fastener would tend to loosen and cause the integrity of the joint to be at risk.
The line was placed back into service in March 2021, over six months after the accident after a total of 37,500 hours of repair and environmental protection work. Some 30,000 tonnes of soil had been removed from the accident site.
Mr Simon French, the Chief Inspector of Rail Accidents, said that there was “a failure to appreciate” the importance of the correct fastenings in the braking system. The case study focuses on the fastener related causes of the accident and what lessons can be learned and applied to both rail and other industries.
Further details on the online training course are available here.