At 23:20 hrs on 9 March 2008, a ‘low ride’ road rail excavator and two road rail wagons derailed. A ‘low ride’ road rail excavator is a machine that shares its weight between its road and rail wheels during on track operation; the rail wheels guide the machine along the track while the road wheels provide the means of traction and braking.
The excavator derailed after its operator transferred weight from its rail to its road wheels. He did this because the track gradient and overloaded wagons increased the amount of traction he required, while the traction available to him was reduced by:
- rainwater that had reduced adhesion between the rubber road tyres and the steel rails
- uncompensated tyre wear that transferred weight, originally applied through the road wheels, to the rail wheels.
As a consequence of this accident, RAIB has made four recommendations.
One recommendation is targeted at the new owners of the road rail excavator, the Downpatrick and County Down Railway, and requires it to maintain and operate the excavator so that it distributes its weight appropriately between road and rail wheels.
The remaining recommendations are targeted at Northern Ireland Railways and require them to identify and eliminate or mitigate the factors that affect the safe operation of road rail vehicles, brief this process to the members of staff who have responsibility for the safety of vehicle movements on track, and ensure road rail vehicles and their associated wagons are inspected and assessed before permitting operation on their infrastructure.
Response to recommendations:
- RAIB will periodically update the status of recommendations as reported to us by the relevant safety authority or public body.
- RAIB may add comment, particularly if we have concerns regarding these responses.