The Rail Accident Investigation Branch has released urgent safety advice following a freight train derailment in Manchester.
The freight train, which was travelling between Peak Forest and Salford, derailed as it passed over a bridge on the 6th of September 2024.
2 Class 66s were hauling 24 wagons to Salford when it passed through Audenshaw in Manchester. The locomotives and 10 wagons passed over the bridge safely, but the next nine wagons derailed.
The advice is that existing inspections and maintenance regimes may not be enough to find failures of baseplate chair screws, which can lead to a loss in track support in longitudinal timber systems.
During its investigations, the RAIB found that the rails on the bridge were supported by PAN M6 baseplates but were held onto the timber by just 2 LSA chair screws.
13 failed chair screws were recovered from the site, and non were marked with HT, which indicates that they were not made of high-tensile material.
The investigation is continuing.
Responses
A very concerning finding by the RAIB, that baseplate screws were the cause of the derailment on a wheeltimber ( longitudinal timber ) bridge. Whilst it is acknowledged that metal components, when under dynamic loadings are subject to stresses, the most concerning issue is that even if the screws appear to be; visually in good condition, squarely positioned, not leaning, tight and resist movement, there can still be hidden faults, such that the whole rail fixing can literally be held hanging on by a thread.
This leaves the question, how are these screws to be assessed or monitored for their integrity?
There has to be automated detection of wheeltimber bridges, probably using acoustic emission methods of monitoring.
It would be impractical for patrollers to tap every bolt and listen for the sound, whether it rings like a bell for good fixings, or whether the sound is dulled for suspect fixings, like the old days of “Wheel-tapping”.
Having experience of both Non-Destructive Testing and now working in the rail Industry, in my opinion, remote Acoustic Emission monitoring is the way to go.
Acoustic emission is the modern equivalent process of the Wheel-tapper, in that when under dynamic loading of train axles above, the fixings will emit a characteristic sound, this sound will change from the good baseline, if there is a degradation in any of the fixings through wear or fatigue.
The shape of the screws do not lend themselves to be suitable for Ultrasonic testing. Radiography is wholely unsuitable due to the time it would take, the radiation protection requirements and the location of the screws in-situ requiring a complicated setup for exposure. Dye Penetrant and Magnetic Particle methods are unsuitable methods as they can only be used to detect surface breaking defects where the surface can be seen, not practical for a crack or segregation that is hidden by the baseplate.
Eddy Current testing might be a possible method for hand-held operator testing of individual screws, but is not really suitable for continuous remote monitoring of a whole under-bridge’s screws.
The wording of the USA is not clear in defining the issue, which would appear to be undetected cracking in the thread core of the fastening screws used to secure rail baseplates on timbers, with the type of baseplate using just 2 screws which from the description & observations of track types, are normally found on bridges with longitudinal timber waybeams
The description highlights a very challenging detail of the screws fracturing in a concealed place plus a fractured screw still held in such a way that it resists turning when being rotated even when broken
The HT marked screws do offer a lower risk of fracture but perhaps a regime that detects modified vibration when it reflects back from a crack, or the system resonance changes for such sections of track during periodic monitoring
A new testing system?
Was asking about wet bed detection when NMT started running 20 years ago, & there’s sure to clues that can be spotted
Bring back length men far more efficient and cost effective ||!! Than these stupid measurement trains !! Since their introductions the amount of minor track and civil engineering faults that have taken place is frightening and increased to major incidents is very worrying !! Either the time for reporting the incident is to long or is just been ignored !!! Yet another farce and a serious one as well !!!
What planet are you on? They’re trying to get workers OFF the tracks. You seem oblivious to the problem of near misses and the fact that walking miles on ballast leaves people semi-crippled by the time they reach old age. Get real, mate!
England is built on “that’ll do” , regs are there but only on paper. Reality. That’s it.
Looks like the track maintenance team has missed the important detail is the checking of every, I mean every LSA screws are tight. Probably the non High Tensile bolts had/have rotted away. Why were LSA bolts ever made of non tensile steel. They have to be exposed to the elements and are screwed into moisture retaining timber sleepers.
Cut backs on maintenance. This is just the beginning
Read the details
A fractured thread can remain held in line & resists rotation
Fracturing looks to initiate from stress raising root of thread & more study may identify manufacturing tolerances which may amplify detail
Currently also tracking cyclic traction loading on railhead that takes this critical area outside elastic limit to deliver Sqats & Sfuds, perhaps a consequence of very powerful traction working much heavier trains but as few as 12 minuscule contact patches acting on the rails & then the rail fixings …. remember when 88lb rails had 4 screws per chair?