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We know what was wrong with Ground Frame 2B – but can RAIB establish why?
Tilting around a left hand curve at close to its Enhanced Permitted Speed of 95 mile/h, Virgin West Coast Pendolino No 390033 derailed at Points 2B in the Lambrigg Ground Frame between Oxenholme and Tebay on the West Coast main Line.
As at Potters Bar five years earlier, work on the points had left the left hand switch blade bowed inward, reducing the width of the flangeway. With 30,000 wheels a week through the points at speed, the resulting high frequency impacts, as flanges squeezed through the narrowed gap, created vibration which could either fatigue the stretcher bars holding the switch blades in place or loosen their securing nuts.
And at around 20.15 on 23 February, the last nut came off its thread, or a fatigue crack took the remaining intact stretcher beyond the tensile limit of the remaining steel. The left hand stretcher bar snapped across to its stock rail, setting that side of the points towards a 20mile/h turnout.
Whether this happened following the passage of the previous Pendolino, or after the first bogie of 390 033 had passed through points 2B remains to be confirmed. The Rail Accident Investigation Branch (RAIB) believes the former; Informed Sources still think that the latter could be the case.
As the left hand wheels of the first bogie to hit the failed points were diverted to the right, the right hand wheels ran normally. As the gauge narrowed, and aided by the heavy rain lubricating the interface, the wheels climbed over the rails, launching the leading vehicle into the air.
It landed with its right hand wheels running in the four foot of the adjacent track. But then, at the second half of the crossover, the right hand wheels met the facing switch blade of the points 2A.
It seems likely that this swung the driving car back to the left The abrupt movement ruptured the coupling to the second vehicle allowing the rotation induced by the switch blade to continue. By now the track had emerged from a cutting onto an embankment down which the still turning driving car plunged, ending up on its right hand side facing backwards.
Newton 's law about action and reaction meant that the force which ruptured the coupler turned the leading end of the second vehicle at right angles to the track. Still slowing it slid broadside on until it hit an overhead line support mast and slid down the embankment to the left. It came to rest at right angles to the track on the embankment with the leading end in the air.
Meanwhile coaches three, four five and, perhaps, six had followed the second vehicle onto the adjacent up track. But with the cess rail torn up by the leading vehicles there was nothing to stop them following the second coach down the embankment.
Carriages three four and five ended up on their sides at the foot of the embankment, with the remaining vehicles, lying along the side of the embankment, leaning over from the vertical. Other than the first and second vehicles, the coaches remained coupled together.
There is an alternative scenario, where the points fail either under the driving car or immediately before the first bogie of the second coach. This would have had the effect of pushing the rear of the driving car to the right, starting the jack-knife motion which broke the coupling. I quote this possibility for completeness. Truth marks on the backs of the wheel flanges should provide the answer.
RAIB's first Interim Report says that the train came to rest around 600m beyond the points which initiated the derailment. From the moment of derailment at Points 2B, it would have taken under 25 seconds for the train to stop.
One person died as a result of the accident and 22 were taken to hospital. Five people remained in hospital when the Rail Accident Investigation Branch Published its first Interim Report on February 26, three in a critical condition.
RAIB, handling its first major accident, moved with commendable speed, publishing its Interim Report on 26 February. This confirmed that the points were the immediate cause of the derailment. It added that ‘no evidence has been found to date that indicates the driving of train, the condition of train or the signalling control system contributed to the accident'.
Network Rail Chief Executive, John Armitt personally responded to the Report's findings. ‘Network Rail is devastated to conclude that the condition of the set of points at Grayrigg caused this terrible accident. We accept the RAIB report in all respects. We would like to apologise to all the people affected by the failure of the infrastructure'.
By then Network Rail had already completed over 700 special inspections, covering 900 similar points using 113 lb/yard rail. ‘Nothing out of the ordinary' was found according to Mr Armitt.
This is what the RAIB Report, which you can find in Professional Stuff on Alycidon Rail ( www.alycidon.com ), said about the condition of points 2B.
27. Investigation of the lock and stretcher bars in the facing points at Lambrigg crossover showed that one of three stretcher bar was missing, and bolts that secured the lock bar and another stretcher bar were not in place – some of these bolts and the associated nuts and washers were found in the ballast, but others were not. However, the RAIB search of the area has not been completed. There is no evidence that the bolts had been wrenched free . Two of the stretcher bars were fractured; in one case the nature of the fracture surface indicates that it may have been consequential to the derailment. In the other case, the fracture surface indicates that it may have predated the derailment. The latter will be confirmed by further analysis.
28. There was therefore no complete stretcher bar in place between the switch rails immediately before the derailment. The left hand switch rail was free to move across close to the left hand stock rail whilst the right hand switch rail remained, correctly, against the right hand stock rail.
RAIB Interim Report 26 February 2007 |
Note that RAIB is vague about the specific condition of individual stretcher bars. It turns out that this is deliberate obfuscation, because when I asked for clarification, it was refused. However, I was referred to Paragraph 28 which says that none of the stretcher bars were functioning.
Note that Paragraph 28 identifies this condition as ‘immediately before the derailment'. In other words RAIB believes the last stretcher bar let-go immediately after the passage of the preceding Pendolino.
Meanwhile, here is Informed Sources own analysis of the condition of Points 2b.
Points 2B - conditionThis set of points has three permanent way (P-Way stretchers) stretcher bars holding the moving switch blades the correct distance apart. A fourth stretcher bar (the lock stretcher) provides an attachment point for the rods which detect that the points have been closed correctly when being re-set. Unlike the points at Potters Bar, which used UIC 60 rail, Lambrigg 2B was of an older design using British Rail pattern 113lb/yard rail. The stretcher bars are fabricated from flat steel strip. A right angle bend at each end allows the stretcher to be clamped to the switch blade with a pair of nuts and bolts. In the more modern design installed Potters Bar the P-Way stretchers are simple steel rods with threaded ends. The ends of each rod pass through brackets attached to the switch rails. The rods are then clamped in place by nuts and lock nuts on the threaded section on each side of the bracket, allowing the stretchers to be adjusted. Looking along Points 2B in the direction of travel the stretcher bars and their condition after the derailment is as follows: 1 Lock stretcher. Both nuts on left hand side undone. 2 Front stretcher One nut undone on left hand side and stretcher bar fractured close to mounting point. 3 Intermediate stretcher. Removed. Some bolts still in place. 4 Rear stretcher. Fractured at right hand end where the bar bends through a right angle and with the nuts also undone on this side
According to the RAIB interim report, one of the fractures may have pre-dated the accident and the other may have been the result of an impact during the derailment. RAIB notes that some, but not all bolts nuts and washers were found lying on the ballast. |
As at Potters Bar, the condition of the nuts suggests that the Lock Stretcher was the last to fail, after the high frequency forces of passing wheels vibrated the remaining nut loose. On the front stretcher the condition of the fracture suggests that it was not caused during the accident.
RAIB forensic experts will be looking very closely at the build up of rust and other debris at the points where the missing Intermediate Stretcher was clamped to the switch blades. Development of rust suggests that the left hand nuts had been removed more recently than those on the right hand side. When this stretcher was removed is, of course, the key issue.
What we do know is that the New Measurement Train (NMT) made a routine inspection run over Points 2B on 21 February. Subsequent examination of the video recording showed that the intermediate stretcher was already missing, but that the nuts were in place on the other stretchers.
Several readers have e-mailed to ask why the NMT did not detect and report something so obvious as a missing stretcher bar. This is to misunderstand both Network Rail's inspection philosophy and the role of the NMT.
What the yellow IC125 does is inspect plain line. The video, and the associated image recognition hardware, is there to look for displaced or missing track clips.
This is no easy task, because there are 16 different types of track fastening on the network. On the French high speed lines I am told that the equivalent of the NMT has to cope with a single type of clip.
Because points are both more complicated and involve judgement they are not suitable for automatic inspection. They are subject to a regime of weekly visual inspections, monthly detailed inspection and test and annual overhauls.
Lambrigg 2B had its last monthly inspection on 3 February, with the subsequent visual inspection made on 11 February. But RAIB says that ‘there is evidence' that the next visual inspection scheduled for February 18 did not take place.
There are unconfirmed reports that Points 2A were used for a tamper movement on 15 February. But at the time of the Interim report, RAIB has not been able to ‘locate evidence of the last use of the Lambrigg 2B points'.
Lambrigg 2A and 2B are known as a ‘ground frame'. Such points are controlled locally to allow trains or track plant to move between the up and down lines, for example during maintenance.
Originally manually operated, the points were converted to power operation by electric point motors, with the controls housed in a cubicle beside the track. The local controls are kept locked remotely and can be released only by a signaller in the Carlisle power box.
To move the points, the operator at the trackside has to contact Carlisle box and request authorisation. When permission is given, the interlocking sets the automatic signals on either side of the section containing the ground frame at danger and the controls are released.
Following the move, the points are reset to normal and Carlisle box notified. Local control is locked out of use and the semi-automatic signals returned to green
All this suggests that inspection reports, the log in Carlisle Power Box and associated recorded messages should provide a timeline of inspection and use and pin down when the intermediate stretcher was removed.
A theory going the rounds is that the points were dismantled to allow a move without notifying Carlisle power box. Unless this was done during a possession, it would be a suicidal activity on a 95mile/h curve.
On top of which this would have meant moving the right hand swtchbIade away from its stock rail sufficently to allow a flange through. Points are set up with a detection tolerance of under 5mm at the switch blade tips.
In addition to disconnecting the drive, the detector rods would have to be detached from the lock stretcher and then reassembled after the move to the original detection tolerance. A lot of trouble and risk to avoid a simple phone call.
Meanwhile, there are two worrying alternatives facing Network Rail. The first is that the intermediate stretcher was missing for some time and that its absence was not reported during inspections. Bolts loosened by the vibration could have been tightened up. This should show up in the audit trail of inspection records
Equally worrying is the possibility that the stretcher bar went missing after the 11 February inspection. In this case the nuts could have vibrated loose very quickly.
By chance, I was talking to two manufacturers of specialised nut locking systems at the Railtex Exhibition the day before the accident. While one design is being supplied in quantity for rod-type stretcher bars, fastenings on older switches and crossings using nuts and bolts appear to have retained traditional fastening methods.