RMweb Gold martin_wynne Posted July 25, 2013 RMweb Gold Share Posted July 25, 2013 systems such as TVM430 as used on French TGV lines / HS1 will enforce a maximum speed regardless of what a driver does and it is simply impossible for a train to enter said restriction above the specified speed. Even in the event of a brake failure? "Impossible" is a dangerous word to use in the context of safety. Martin. Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 Even in the event of a brake failure? "Impossible" is a dangerous word to use in the context of safety. Martin. Train brakes do not fail into an 'off' state. Any failure within the system causes the brakes to be fully applied, it takes the action of a pressurised air line to keep the brakes off. So if any airline or indeed the compressor fails the brakes are applied. Its quite clever when you think about it :-) Link to post Share on other sites More sharing options...
dave_long Posted July 25, 2013 Share Posted July 25, 2013 I'm with Miss prism, the carriage seems to be at a different height before the curve is encountered. Link to post Share on other sites More sharing options...
RMweb Premium phil-b259 Posted July 25, 2013 RMweb Premium Share Posted July 25, 2013 Even in the event of a brake failure? "Impossible" is a dangerous word to use in the context of safety. Martin. The only way the 'impossible' could occur in the sense of an overspeeding train under the supervision of the TVM430 signalling system is if the signalling system had the wrong speed limits programmed into it at the design stage and for that mistake not to be picked up during the extensive testing all signalling systems require. Link to post Share on other sites More sharing options...
Mike at C&M Posted July 25, 2013 Author Share Posted July 25, 2013 I'm with Miss prism, the carriage seems to be at a different height before the curve is encountered. You must remember that these units have power cars and generator cars with higher roofs than the 'Talgo' stock. So there may be a degree of optical illusion to consider. However, I also agree that the train appears to derail behind the leading power car. Also, as has been touched upon earlier, this is the first restrictive curve at the end of the high speed line, so the train will have been legitimately running at high speed not long before the derailment. Link to post Share on other sites More sharing options...
Glorious NSE Posted July 25, 2013 Share Posted July 25, 2013 Comparing the overhead here: http://www.bbc.co.uk/news/world-europe-23449336 with the video clip, the transition to the curve starts around the bridge, and from the video the train does not appear to be derailed at that point. - I would gauge the point of derailment to be somewhere around the white dot marked 'direction of travel' on the BBC overhead shot, that points it neatly at the end of the retaining wall, and seems to be where the curve starts reaching it's normal radius... I think what we're seeing in the vid is the generator car (possibly due to a higher centre of gravity?) trying to roll over on the curve due to centrifugal force, that then pulls the following coaches behind it and drags the rear of the driving car off... I don't follow that it would become unstable due to braking unless for some reason the front was braking extremely hard, the rear was not braking, or was even powering, and the train couldn't handle that force.....which i'd have thought would be a very unusual set of circumstances for a modern 'unit train' set, especially one that is largely articulated... Link to post Share on other sites More sharing options...
RMweb Gold martin_wynne Posted July 25, 2013 RMweb Gold Share Posted July 25, 2013 Train brakes do not fail into an 'off' state. Any failure within the system causes the brakes to be fully applied ANY failure? Including any failure, fault or bug within the electronics, control system and software? I don't believe it. I think it is very dangerous in planning safety systems to assume a thing is "impossible". The likelihood may be vanishingly small, but it can't ever be impossible. Martin. Link to post Share on other sites More sharing options...
RMweb Gold martin_wynne Posted July 25, 2013 RMweb Gold Share Posted July 25, 2013 As I understand it, these wheels are not fixed on an axle, to allow change of gauge. How do they behave under emergency braking if opposite wheels are not constrained to rotate at the same speed? And on a sharp curve? Martin. Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 Comparing the overhead here: http://www.bbc.co.uk/news/world-europe-23449336 with the video clip, the transition to the curve starts around the bridge, and from the video the train does not appear to be derailed at that point. - I would gauge the point of derailment to be somewhere around the white dot marked 'direction of travel' on the BBC overhead shot, that points it neatly at the end of the retaining wall, and seems to be where the curve starts reaching it's normal radius... I think what we're seeing in the vid is the generator car (possibly due to a higher centre of gravity?) trying to roll over on the curve due to centrifugal force, that then pulls the following coaches behind it and drags the rear of the driving car off... I don't follow that it would become unstable due to braking unless for some reason the front was braking extremely hard, the rear was not braking, or was even powering, and the train couldn't handle that force.....which i'd have thought would be a very unusual set of circumstances for a modern 'unit train' set, especially one that is largely articulated... I agree the generator car appears to pull both the carriage rake and driving car off. I also agree that there ought not be massive breaking force on the couplings, train brakes do not work like the brakes on you car and trailer or indeed caravan. Every axle is braked and every vehicle has more than enough braking to stop itself, braking forces transmitted through the coupling or buffers ought to be minimal. I can not see the rear powering either being likely but thats a little more probable than braking causing the derailment. Still very very unlikely though. Link to post Share on other sites More sharing options...
Adrian Wintle Posted July 25, 2013 Share Posted July 25, 2013 I think what we're seeing in the vid is the generator car (possibly due to a higher centre of gravity?) trying to roll over on the curve due to centrifugal force, that then pulls the following coaches behind it and drags the rear of the driving car off... I'd agree. It looks like the rear of the generator car jumps up - possibly the rear bogie derailing? The power car definitely holds the rails until pulled off by the derailed cars behind it. Adrian Link to post Share on other sites More sharing options...
Miss Prism Posted July 25, 2013 Share Posted July 25, 2013 Comparing the overhead here: http://www.bbc.co.uk/news/world-europe-23449336 with the video clip, the transition to the curve starts around the bridge, and from the video the train does not appear to be derailed at that point. - I would gauge the point of derailment to be somewhere around the white dot marked 'direction of travel' on the BBC overhead shot, that points it neatly at the end of the retaining wall, and seems to be where the curve starts reaching it's normal radius... I think you're right about the lateral point of derailment, Martyn, but the commencement of the lifting of the vehicle, which should perhaps be considered to be the start of the 'derailment', looks to me to be far closer to the motorway bridge (and who knows, possibly before it). Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 ANY failure? Including any failure, fault or bug within the electronics, control system and software? I don't believe it. I think it is very dangerous in planning safety systems to assume a thing is "impossible". The likelihood may be vanishingly small, but it can't ever be impossible. Martin. Read up on how they work first! Maybe every single valve on every cylinder could seize into the charge position and both the electronic control of the brakes and the mechanical control of the air valves fail into a lock the brakes off state but you are not talking about a single failure but hundreds! If you really want to chase the tail of the probability curve then yes, the brakes may fail into a locked off state but thats as likely as you researching what you are talking about and coming back with a more informed opinion. Link to post Share on other sites More sharing options...
RMweb Premium phil-b259 Posted July 25, 2013 RMweb Premium Share Posted July 25, 2013 ANY failure? Including any failure, fault or bug within the electronics, control system and software? I don't believe it. I think it is very dangerous in planning safety systems to assume a thing is "impossible". The likelihood may be vanishingly small, but it can't ever be impossible. Martin. The railways industry is very well aware of bugs / mechanical failure and thus everything is designed in such a way as to negate them. If fact the most dangerous bit of equipment is the human being at the controls as we cannot 'design' its behaviour or 'force' its compliance with a particular set of conditions (and driving a train IS basically following a set of specific instructions given by the signalling system etc, its not an environment for people who want to be creative) Link to post Share on other sites More sharing options...
RMweb Gold martin_wynne Posted July 25, 2013 RMweb Gold Share Posted July 25, 2013 but that's as likely as you researching what you are talking about and coming back with a more informed opinion. It's possible to tell me I'm wrong without resorting to personal abuse. Why do RMweb discussions so often reduce to this? Martin. Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 It's possible to tell me I'm wrong without resorting to personal abuse. Why do RMweb discussions so often reduce to this? Martin. I merely thought a real world example of the statistical likelyhood would be a better example than basing my response on facts; facts backed up with evidence on here are about as statistically likely as a trains brake system failing. Anyway have a read here, its a little basic but it examples the crux of how they work well enough https://en.wikipedia.org/wiki/Railway_air_brake Link to post Share on other sites More sharing options...
Mod3 Posted July 25, 2013 Share Posted July 25, 2013 The discussion has so far been generally sensible however it has been noted that some unnecessary arguments are raising their heads - let's stick to known facts or reasonable discussion please otherwise the thread will be locked off. Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 You're right. Sorry. Link to post Share on other sites More sharing options...
RMweb Gold The Stationmaster Posted July 25, 2013 RMweb Gold Share Posted July 25, 2013 From a Google map, I estimate the radius of the curve that the train is entering to be approx 350m. A 200mm superelevation will provide 50mph (80kmph) optimisation on such a radius, but I doubt that degree of superelevation would be present, so we are probably looking at a 'cant deficiency' situation for that curve anyway. (Someone please check my sums!) One of the drivers is reported to have made a phonecall shortly before the crash saying he was doing 190kmph (approx 120mph). The track before the curve (beyond the motorway/autoestrada bridge) is substantially straight. Looks to me as though the derailment is beginning before the curve is encountered, so I'm inclined to go along with Nth's idea of an emergency brake application causing (somehow) the leading passenger coach to lift and thereby (centripetally) wanting to carry straight on when the the train does encounter the curve. That would certainly look to be a reasonable hypothesis in view of the point at which the initial derailment appears to take place although it does depend on proportional braking rates over the various vehicles (but I would be surprised if greater brake force was NOT on the power vehicles. And it wouldn't be the first derailment mechanism to have worked like that - but we are still in the area of conjecture of course. Interestingly the presence of articulation does look as if it kept the coaches in line - until they came up against the immovable concrete of the retaining wall. Link to post Share on other sites More sharing options...
RMweb Premium NGT6 1315 Posted July 25, 2013 RMweb Premium Share Posted July 25, 2013 I'd agree. It looks like the rear of the generator car jumps up - possibly the rear bogie derailing? The power car definitely holds the rails until pulled off by the derailed cars behind it. Each of the generator cars has one conventional bogie on the outer end which faces the power unit, while the inner ends sit on the outermost Talgo running gears. Have a look here: http://upload.wikimedia.org/wikipedia/commons/f/ff/RENFE_Class_730_Viaducto_Martin_Gil.jpg (Link to full size image posted to allow zooming in if required.) Link to post Share on other sites More sharing options...
RMweb Premium phil-b259 Posted July 25, 2013 RMweb Premium Share Posted July 25, 2013 That would certainly look to be a reasonable hypothesis in view of the point at which the initial derailment appears to take place although it does depend on proportional braking rates over the various vehicles (but I would be surprised if greater brake force was NOT on the power vehicles. And it wouldn't be the first derailment mechanism to have worked like that - but we are still in the area of conjecture of course. Interestingly the presence of articulation does look as if it kept the coaches in line - until they came up against the immovable concrete of the retaining wall. Interesting - but then we have seen this sort of behaviour before. Articulation is good at keeping things in line until they have stopped but it also means that if the lead vehicle hits an obstruction, so will all those following behind. Also it follows that while articulation might be good at keeping vehicles upright, once one vehicle topples over articulation encourages subsequent vehicles to follow suit Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 That would certainly look to be a reasonable hypothesis in view of the point at which the initial derailment appears to take place although it does depend on proportional braking rates over the various vehicles (but I would be surprised if greater brake force was NOT on the power vehicles. And it wouldn't be the first derailment mechanism to have worked like that - but we are still in the area of conjecture of course. Interestingly the presence of articulation does look as if it kept the coaches in line - until they came up against the immovable concrete of the retaining wall. In normal every day braking I think there is more braking coming from the power vehicle, as it will lean to using regenerative braking if it has it and avoid using air in the trailers, but in a full on emergency stop I think the limit is the friction between rail and wheel, which is simply a function of mass of the vehicle. So the heavier vehicles will have proportionally more grip to their mass but will have more mass to stop, thus I think (and I dont know this) that there ought not to be any significant brake force distributed through the train. This assumes the trailers all have enough brakes to use all available friction and the emergency brake propagated through the train quickly. Link to post Share on other sites More sharing options...
RMweb Premium NGT6 1315 Posted July 25, 2013 RMweb Premium Share Posted July 25, 2013 The class 130, 730 and also the standard gauge only class 102 and 112 sets use coaches derived from the Talgo 7 generation. ISTR these are equipped with electronically controlled, hydraulically actuated brakes rather than pneumatic brakes, which in turn are connected to the power units' busbar by way of relay valves. Link to post Share on other sites More sharing options...
Miss Prism Posted July 25, 2013 Share Posted July 25, 2013 Damian - not wishing to put words in your mouth, but are you saying that, owing to the differences of mass between the vehicles in the train, the likelihood of the brakes applying an identical deceleration to each vehicle is not great, especially in an emergency brake application? Link to post Share on other sites More sharing options...
damian Posted July 25, 2013 Share Posted July 25, 2013 The class 130, 730 and also the standard gauge only class 102 and 112 sets use coaches derived from the Talgo 7 generation. ISTR these are equipped with electronically controlled, hydraulically actuated brakes rather than pneumatic brakes, which in turn are connected to the power units' busbar by way of relay valves. I think they are air controlled hydraulic brakes, the control system is broadly similar to air brakes but with an hydraulic system to do the 'heavy lifting' because of the greater force available to hydraulics over air. Link to post Share on other sites More sharing options...
RMweb Gold TheSignalEngineer Posted July 25, 2013 RMweb Gold Share Posted July 25, 2013 ................... One of the drivers is reported to have made a phonecall shortly before the crash saying he was doing 190kmph (approx 120mph). The track before the curve (beyond the motorway/autoestrada bridge) is substantially straight. ........................... Again working from Google Earth and the video, the distance between the motorway and the road bridges is about 240 metres. The train covers this in about 5 seconds, including sliding along the wall, which gives an average speed of about 170 kilometres per hour. The new high speed line runs for about 80 km before joining the old alignment approaching the motorway bridge. Link to post Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.