At about 08:20 hrs on 9 March 2020 the wagon left the works and continued to run away on the descent towards Horrocksford Junction, where the line to the cement works connects to the main railway line from Blackburn to Hellifield.
As it ran away, the wagon broke through the gates at the exit from the cement works and ran over an open level crossing located on a public road, causing two cars to stop.
A short distance later, the wagon passed over an open crossing on a private road. Neither level crossing had its manually activated warning equipment switched on before the wagon passed over.
On reaching Horrocksford Junction, the wagon derailed on the trap points which worked as intended to prevent such unauthorised movements from reaching the main line.
The derailed wagon stopped clear of the main line and no trains were nearby when the derailment occurred. The wagon ran away for a total distance of about 1.2 km.
RAIB said: “Our investigation will seek to identify the sequence of events which led to the accident. It will also consider any factors relevant to the performance and design of the wagon’s braking system, how the risk of runaway wagons from within the cement works was controlled and any relevant underlying factors.”