Collapse investigation calls for improvements in Network Rail inspections

The last six tank wagons of a ten-wagon freight train carrying fuel derailed as the train crossed a bridge south of Stewarton in January 2009. The bridge collapsed under the train and the derailed wagons overturned. Fuel from four of the wagons leaked into watercourses, causing severe environmental damage. There were also localised fires.

RAIB found that the immediate cause of the derailment was the collapse of the bridge following a catastrophic failure of its east and centre main girders. Heavy corrosion had so significantly weakened these main girders that they were no longer able to carry the loading from the heaviest trains that were permitted over the bridge.

The form of construction of the bridge led to a hidden corrosion trap that affected the inner surfaces of the main girders, with the result that holes formed in them. Incorrect assumptions about the thicknesses of the main girder web plates had been made during routine inspections and no allowance was made for material loss due to corrosion. No arrangements were made to inspect the hidden parts of the bridge girders where the heavy corrosion was occurring, the report found. The heavy corrosion would have been revealed if full repairs and painting had been carried out when the bridge was waterproofed in 1987.

Main girder web corrosion had been identified during the last routine detailed examination in October 2003 but there was a lack of action in response to the issues. There was also a lack of any response to the continued reporting of corrosion defects identified in the routine annual visual examinations.

The bridge was due for replacement as part of an upgrade scheme and RAIB found that this resulted in information that was potentially relevant to its condition not being considered.

RAIB has targeted 11 recommendations at Network Rail including checks and remedial action with regard to other bridges that may be at risk because of similar hidden corrosion issues or erroneous assessment findings. Improvements are needed in Network Rail's methods and processes for the examination of hidden critical parts of structures, it says, as well as improvements in bridge assessement procedures and in the management of the information used for making decisions about structural safety.