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3 December 2020 Derailment investigation Dr Mark Burstow, Network Rail Technical Fellow Principal Vehicle Track Dynamics Engineer Providing technical leadership Overview • History- when and how did derailment investigation really develop? • Causes of derailment and contributory factors • The investigation process o Evidence collection o Analysis Derailment Investigation December 2020 Page 2 How derailment investigation became a science The 1960s became the era of the freight train derailment • Particularly, short wheelbase 4-wheeled wagons on plain track • A worrying trend o 11/2/1961: Between Rugby & Lutterworth o 1/4/1963: Weedon o 21/1/1966: Steventon o 31/7/1967: Thirsk 450 400 Passenger 350 Freight 300 250 200 Derailments Derailments 150 100 50 0 1950 1960 1970 1980 1990 2000 2010 2020 Year Derailment Investigation December 2020 Page 3 Vehicle dynamics research Newly formed BR Research department took on challenge of finding cause for these derailments • Developed theoretical and mathematical basis for wheelset dynamic instability (‘hunting’) • Identified that some freight vehicles were becoming unstable at speeds as low as 20mph, depending on condition of suspension and wheel wear Understanding of vehicle dynamics provided better explanation of the causes of derailment • Separate group within BR Research formed to undertake investigations • Became part of AEA Technology Rail following privatisation, then DeltaRail Derailment Investigation December 2020 Page 4 So, where do we come in? Following the Hatfield accident in October 2000 • Railtrack created its own ‘in-house’ expertise in vehicle dynamics and vehicle/track interaction o Initially to investigate rolling contact fatigue (RCF) o Support the cross-industry Vehicle/Track Systems Interface Committee (V/T SIC) In 2010 DeltaRail announced they would withdraw from track consultancy work • Including supporting derailment investigation (‘call-off’ contract with NR) In 2011 Head of Track asked if NR’s wheel/rail interface team could provide a similar service Derailment Investigation December 2020 Page 5 Our first derailment Edinburgh Princes Street Gardens, 27th July 2011 Derailment Investigation December 2020 Page 6 Where have we been since then? 43 incidents in 9 years • Roughly 1 every 2½ months Derailment Investigation December 2020 Page 7 Where have we been since then? 43 incidents in 9 years • Roughly 1 every 2½ months But it’s not as simple as that • October 2013: 2 Incidents on the same day Derailment Investigation December 2020 Page 8 Where have we been since then? 43 incidents in 9 years • Roughly 1 every 2½ months But it’s not as simple as that • October 2013: 2 Incidents on the same day • January 2013: 4 incidents in 1 week Derailment Investigation December 2020 Page 9 Where have we been since then? 43 incidents in 9 years • Roughly 1 every 2½ months But it’s not as simple as that • October 2013: 2 Incidents on the same day • January 2013: 4 incidents in 1 week And we’ve even been to the same place twice! • April 2014 • June 2015 Derailment Investigation December 2020 Page 10 So, how does it work? Notifications of incidents obtained from National Operations Centre • Liaise with the Route/Region team regarding attendance Sometimes we get invited some time after the event • Not ideal! Our purpose • Impartial investigation • Evidence collection • Technical analysis to explain the mechanism and course of derailment • Assist any formal investigation with technical input What we don’t do.. • Find a scapegoat! • Lead/chair investigation panels • SPADs, collisions, level crossing incidents… We are not in competition with, or replace, RAIB • Usually work together • RAIB may ask for our assistance: at site or with analysis Derailment Investigation December 2020 Page 11 The investigation process: 1- mobilise! Derailment Investigation December 2020 Page 12 The investigation process: 1½- arrive! Derailment Investigation December 2020 Page 13 The investigation process: 2- evidence collection Two crucial questions to answer • What happened? • How did it happen? Derailments very rarely (if ever) have a single cause • Result of a combination of vehicle and track contributory factors Derailment Investigation December 2020 Page 14 What are the mechanisms of derailment? Gauge spread Flange climb S&C Derailment Investigation December 2020 Page 15 1. Flange climb derailment Process whereby a wheel climbs up the side of the rail and the flange tip runs across the head of the rail to drop onto the field side of the rail Flange climb is a risk when lateral forces (Y) on the wheel overcome the vertical forces (Q) • Quantified by Nadal’s limit Wheel • Nadal’s limit gives critical Y/Q ratio: Y/Q above this Wheel load, Q limit indicate derailment is a risk Rail • Nadal’s limit depends on Lateral force, Y Normal (reaction) o wheel/rail contact angle force, N Tangential (reaction) o friction between wheel and rail force, F Contact angle, q Derailment Investigation December 2020 Page 16 How do we get a high Y/Q? Increase ‘Y’ (lateral) force • Tight radius curve • Lateral alignment track faults/variations in combination with tight radius curve Decrease ‘Q’ (vertical force) • Track twist • Slow speed- cant excess • Vehicle loading • Vehicle suspension o Twisted bogie frame Many derailments occur on twist faults on tight radius curves with freight vehicles at low speed • Combines many of the features above! Derailment Investigation December 2020 Page 17 2: Complete wheel unloading- Cyclic top Vehicle dynamic behaviour is excited by the top profile of the track • Derailment becomes a risk when vehicle’s natural ‘bounce’ or ‘pitch’ frequency matches, or comes close to, the frequency of dips in the rail • Depends on wavelength of track features and vehicle speed Excitation of vehicle in vertical plane can fully unload the wheels • Wheel ‘bounces’ onto or over the rail More likely as speed increases as there is more energy to drive the behaviour Derailment Investigation December 2020 Page 18 Cyclic top track geometry and vehicle response The response of each vehicle varies with wavelength • The wavelength at which the most unloading occurs varies with speed, suspension design and loading condition • The maximum amount of wheel loading also varies 0.9 Freight 1 (Laden) Freight 1 (Tare) ) 0.8 Q / Freight 2 (Laden) Q 0.7 Freight 3 (Laden) 0.6 Freight 3 (Tare) Freight 4 (Laden) 0.5 Freight 4 (Tare) 0.4 Freight 5 (Laden) 0.3 Freight 5 (Tare) 0.2 Wheel unloading Wheel unloading (Delta 0.1 0 40 50 60 70 80 90 Speed/mph Derailment Investigation December 2020 Page 19 3: Gauge spread As curve radius reduces • Lateral forces on leading wheelset increase and act to push the rails apart • Derailment occurs when sleepers/fastenings are no longer able to support the lateral forces • Usually outer rail of curve moves, allowing wheel on inside of curve to drop in to derailment 50 45 40 High PYS, Wset 1 35 High PYS, Wset 2 30 Low PYS, Wset 1 25 Low PYS, Wset 2 20 15 Gauge spreading Gauge force/kN 10 5 0 0 500 1000 1500 2000 Curve radius/m Derailment Investigation December 2020 Page 20 4. Derailments on switches & crossings (S&C) Switches & crossings provide a real ‘assault course’ for a railway vehicle • Need to provide guidance • Rapid changes in curvature, alignment and rail profiles Different wheel profiles make contact with the switch at different points • Although some wheels can negotiate the switch ok, others may result in the wheel flange lifting on to the top of the switch rail New wheel profile Worn wheel profile Derailment Investigation December 2020 Page 21 Poor switch rail profiles Worn profile of switch blade shows a shallow angle • High friction (poor lubrication) and low contact angle reduces Nadal’s limit (Y/Q limit) • Easier for a vehicle to generate a Y/Q which exceeds Nadal’s limit and flange climb 0 700 500 300 100 800 600 400 200 Derailment Investigation December 2020 Page 22 Derailments on S&C: Crossing issues Guidance at crossings • Crossing nose geometry and poor track quality: allows wheel to climb onto crossing nose Derailment Investigation December 2020 Page 23 Challenges on site Derailment Investigation December 2020 Page 24 The investigation process: 2- evidence collection How do we separate damage as a consequence of derailment from that which was causal? Derailment Investigation December 2020 Page 25 Point of derailment: flange climb marks Derailment Investigation December 2020 Page 26 Flange climb marks: what can we learn Long flange mark: Short flange mark: Low lateral forces acting- flange climb due to reduced High lateral forces forced wheel over rail vertical load: possible track twist or vehicle imbalance? Derailment Investigation December 2020 Page 27 Point of derailment: wheel tread corner marks Derailment Investigation December 2020 Page 28 Sources of evidence: track Develop a keen eye for detail! It can sometimes be useful when parts of the train are blocking access to the PoD Derailment Investigation December 2020 Page 29 Sources of evidence: damage to track Broken rail- cause or effect? It’s vital that you find all the ‘bits’! Derailment Investigation December 2020 Page 30 Absence of evidence? Derailment Investigation December 2020 Page 31 Sources of evidence: damage to track components How many wheelsets derailed and where did they run? Derailment Investigation December 2020 Page 32 Sources of evidence: track geometry Derailment Investigation December 2020 Page 33 Sources of evidence: rail profiles Direction of travel Sleeper 7 Sleeper 20 Point of derailment Sleeper 50 Sleeper 100
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