A UK government department has taken the unusual step of issuing an official “Urgent Safety Advice” following the derailment of an aggregates train in Manchester in September. The warning is the first such directive of the year. It comes while the official investigation into the accident is still underway. The safety critical level of concern has prompted the immediate warning.
A massively disruptive derailment prompted the call for bridge inspections across Britain. Following the derailment in the Audenshaw neighbourhood of Manchester, the UK government’s accident watchdog has warned that other bridges could be susceptible to the same circumstances. That has led to the issue of an Urgent Safety Advice, recommending the immediate inspection of similar structures around the network.
Duty holders take urgent steps
The Rail Accident Investigation Branch is still working on the accident. However, they have issued a warning that existing inspection and maintenance regimes may not be sufficient to detect the failure of critical components. In certain quite common bridge types, “baseplate chair screws” could fail, which can lead to a loss of lateral track support in longitudinal timber systems. The screws, holding track in place on certain common bridge designs with timber elements, could be in danger of a similar failure to the one that badly derailed an aggregates train in September.
The RAIB has recommended that all ‘duty holders’ should take urgent steps to consider and mitigate this risk. Those responsible parties principally include the national infrastructure agency Network Rail, upon whom the greatest burden of care will fall. The directive also covers other infrastructure managers, and those companies responsible for maintaining or inspecting longitudinal timber systems.
Gauge spread and broken screws
The accident that prompted the warning happened in the late morning of 6 September this year. “At around 11:25, a freight train travelling between Peak Forest and Salford derailed as it passed over a bridge in Audenshaw, Manchester,” said an RAIB statement. “The train involved was made up of two class 66 locomotives and 24 wagons, which were loaded with aggregate. The locomotives and the leading ten wagons passed safely over the bridge. The next nine wagons derailed, with the last of these coming to a stand on the bridge itself.”
No injuries were caused by the accident. However, the derailment caused substantial damage to railway infrastructure and damaged some of the wagons. “The evidence available at this stage to RAIB’s investigation indicates that the derailment was caused by gauge spread within the first half of the bridge, with the track having transferred from ballasted track to a longitudinal timber system as the train entered the bridge,” said the investigators.
Faults almost impossible to detect
The track geometry at this location is quite complicated. According to the RAIB report, the line curves with a radius of approximately 480 metres and an installed cant of around 40mm. The rails on the bridge were supported on “PAN M6” baseplates held to the timber by only two LSA chair screws. The RAIB recovered 13 failed LSA chair screws from the baseplates of the low-side rail. None of these screws were marked with ‘HT’ on their heads, which indicates that they were probably not made of high-tensile strength material.
Preliminary metallurgical analysis of these failed chair screws exhibit signs of what the specialists call both “low-cycle and high-cycle fatigue failure”. A number of them also exhibited fracturing by overload. In most cases, these failures were one or two threads below the shanks, making them almost impossible to see on visual inspection.
Likely disruptions unavoidable
The RAIB says this type of failure has been seen previously in different circumstances. They noted the derailment of a freight train at Sheffield Midland station on 11 November 2020. “The upper portion of a broken screw may still offer some resistance to rotation or removal by hand, which makes detection of an impending failure by gauge widening difficult,” says the RAIB report. “At the site of the Audenshaw derailment, there were no clear indications of significant baseplate, or baseplate packing, shuffle nor timber indentation.”
It seems difficult for any duty holders to take any action other than extensive inspections of at-risk structures. The RAIB has listed at least seven characteristics which may indicate a higher potential risk to longitudinal timber systems using hardwood timbers. These include the use of the PAN M6 baseplates, or similar, with only two chair screws per plate, and the presence of non-HT chair screws, which may be more susceptible to failure by fatigue. The investigators have also recommended vigilance on packing and ballast movements, and a reappraisal of historical instances of local gauge widening under traffic over a short period. If susceptible structures are widespread across the network, there is potential for significant short-term disruption. Maintenance schedules and services could be thrown into disarray, particularly if intensive inspections reveal any urgent remedial action.
