The impact of tilt on keeping a train on the track or do you need tilt to go around a corner faster?

Discussion in 'Traction & Rolling Stock' started by mcmad, 15 Aug 2019.

  1. mcmad

    mcmad Member

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    Mod Note: Posts #1 - #27 originally in this thread.

    Because the track isn't fit for conventional stock to run at 125? Through some minor realignments and such like you might get 115, perhaps a short section of 120 if you push the limits but the only way to get 125 is wholesale reconstruction.
     
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  3. pdeaves

    pdeaves Established Member

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    Tilt exists primarily for customer comfort. The horizontal (track) forces will be the same for tilt and non-tilt. A non-tilt train could zip round the curves if the people on board didn't mind being thrown all over the place.
     
  4. EE Andy b1

    EE Andy b1 Established Member

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    Rubbish!
    Try going round the curves at Weedon, South of Rugby at 125 mph with non-tilt stock and see what field you end up in. Around there it's 100 mph permissible, 110 mph enhanced for 221s and 120 mph enhanced for 390s.
    At 100 mph now it feels like the train wants to leave the rails.
     
  5. Boysteve

    Boysteve Member

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    Can you please briefly explain why? I do not understand why a section of straight track could cope with a 125mph tilting train that would not be tilting at the time anyway, but could not cope with a conventional train at 125mph which cannot tilt anyway.
     
  6. Boysteve

    Boysteve Member

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    I disagree with this, although this may not be the best place in the forum to do it! If a 390 is permitted to round the curve at 120 mph in passenger service then I do not believe any non-tilting stock would end up in a field just 5 mph faster. I appreciate it may 'feel' like that, but the difference in speed between tilting and non-tilting stock ending up in a field is marginal, the main factor being the different positions of the centre of gravity of the train as it rounds the corner (the tilting trains being ever so slightly lower).

    BACK to topic; The discussion digressed over the feasibility of 125mph none tilt running on the Northern WCML. I used the example of Lockerbie to the old Beattock station where very little tilting happens at 125mph on a Pendo', but the 110mph blanket limit remains on none-tilting stock.
     
    Last edited: 15 Aug 2019
  7. LNW-GW Joint

    LNW-GW Joint Veteran Member

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    Weedon has very sharp reverse curves, like Whitmore and Linslade, and it's not surprising you need tilt for 125mph in those and quite a number of similar locations.
    You seem to think the stretch through Lockerbie is "straight" but I'm not sure it is.
    I used to think Nuneaton-Rugby was straight before speed went up to 125mph with tilt, and you can feel the train tilting in places.
    Even the new TV4 route through Lichfield, and the reconstructed section through Rugby station, have full tilt sections.
     
  8. EE Andy b1

    EE Andy b1 Established Member

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    Yep everyone is free to disagree and maybe for another thread.

    The 390 is rounding the curve at 120 mph whilst tilting, shifting the centre of gravity, non-tilt stock can only do 100 mph rounding those curves, so at the 5 mph over EPS speed that you stated, which is 25 mph over permissible speed, i think you would find that (hopefully never happens) that train would be thrown outward off the rails, i don't think the margin is that big, maybe 10-15 mph.

    Remember that train in Spain a few years back!
     
    Last edited: 15 Aug 2019
  9. EE Andy b1

    EE Andy b1 Established Member

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    Just as a side note, Class 390s tilt on EPS equipped routes at speeds above 45 mph, under that they don't tilt.
     
  10. mcmad

    mcmad Member

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    because its not straight track, the curves are around 2000m radius but tighten to 1200m towards Beattock Village. As pdeaves mentions above the rules around speed limits are largely based on passenger comfort, the difference being in the amount of cant deficiency allowed. There are more significant issues around the transition onto the curves as these are what frequently limit the speed as it is not easy or cheap to lengthen the transition for the higher speed.

    For most curve calculations there are 3 limits, normal, maximum and exceptional. the 110/125 EPS split will predominately have the conventional stock running up to the normal limits sometimes up to the maximum, the EPS tilting trains will typically run closer to maximum occasionally towards exceptional.
     
  11. LNW-GW Joint

    LNW-GW Joint Veteran Member

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    There's another issue with 125mph on the WCML.
    TASS is a form of Automatic Train Protection (ATP) which is mandated on new or upgraded lines over 110mph.
    The WCML implemented the speed-supervision side of TASS to meet that requirement, as the line was deemed upgraded with the WCRM project and 125mph line speed.
    The GWML and ECML were already operating at 125mph so didn't have to implement ATP (though the GWML has its BR-pattern ATP).
    It's why class 800s have to run with ATP on the GWML pending its replacement by ETCS.

    I don't know how the MML avoided fitting ATP on its new 125mph sections, maybe TPWS is now considered sufficient - there's also a TPWS+ now.
    So any new trains running on the WCML at 125mph will have to implement TASS for ATP reasons, even if they don't tilt [or ETCS, TASS being a subset].
    It's one of the reasons 91+Mk4s can't use 125mph on the WCML.
    So just allowing the higher speed on straight sections is not the whole story.
    I hope if all this is tosh someone with correct me!
     
  12. Bletchleyite

    Bletchleyite Veteran Member

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    Tilt doesn't substantially shift the centre of gravity, it's more rotation around an axis. It is purely for passenger comfort.
     
  13. EE Andy b1

    EE Andy b1 Established Member

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    So are you trying to tell me a non-tilt train of max speed of 100 mph would not be trying to jump the rails if doing 120 mph on those same curves? Whether it's centre of gravity or rotation around an axis . I'm not saying it would but that's what's trying to happen.
     
  14. Bletchleyite

    Bletchleyite Veteran Member

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    Yes, that is precisely what I am telling you.

    No, it wouldn't. A train tilts by rotating around an axis, the CoG barely moves. It is not like riding a motorcycle.
     
  15. Roose

    Roose Member

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    Believing things isn't that good a basis in the realm of physics unless you bring forward evidence to validate your belief!
     
  16. EE Andy b1

    EE Andy b1 Established Member

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    Sorry for posting this, but this is what can happen.

    A tilting train on a non-tilt speed restriction, speeding! This is the extreme but what my point was could happen.

     
  17. Ianno87

    Ianno87 Established Member

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    AIUI, in general on curves, the speed threshold for passenger comfort is reached far before the speed at which the train would remove itself from the track.

    Trains could go faster around corners if it was a accepted the passengers got flung everywhere.
     
  18. Darandio

    Darandio Established Member

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    Very extreme and very different. Because you were talking about a train trying to jump the tracks at 120mph on a non-tilt 100mph curve.
     
  19. maverickjesus

    maverickjesus New Member

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    That train was doing 110mph+ on a 50mph rated curve, its not even remotely comparable.

     
  20. ainsworth74

    ainsworth74 Moderator Staff Member Moderator

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    Err, that was a train doing 110mph+ in a 50mph speed restriction. A tilting train on tilting track would come off on a bend at that sort of overspeed!
     
  21. Bletchleyite

    Bletchleyite Veteran Member

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    Tilt was irrelevant to that - it was a significant overspeed.
     
  22. EE Andy b1

    EE Andy b1 Established Member

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    I did say that video was extreme.

    So going round a left hand curve and the stock buffeting towards the right, there must be a point where it gives way and wants to leave the left rail, it may not be as low as 20 mph overspeed, but there must be a pivotal point.
     
  23. ashkeba

    ashkeba Member

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    Aren't all tilts rotating around an axis? Is that not what tilting is? The main things not like riding a motorcycle is that the maximum tilt is only 8 degrees and the axis is between two sets of wheels. I do not see why this means the CoG would not move.

    Is there much possibility of more tilting stock for WCML?
     
  24. Hadders

    Hadders Established Member

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    When a train tilts the wheels stay firmly on the rails. The tilt is for passenger comfort.
     
  25. Class172

    Class172 Established Member Quizmaster

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    Strictly speaking yes anything that tilts has an axis of rotation, but it’s important to distinguish the location of the axis. On a motorcycle, the axis of rotation is at the point of contact between the tyres and the road surface, however for a tilting train such as the pendolino, the axis is presumably through the centre of the passenger cabin so is a metre or so off the ground.
     
  26. mcmad

    mcmad Member

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    Of course if you go grossly too fast round a corner any train will derail. Tilt or not doesn't change that point. Has was seen in your example and previously at Morpeth amongst others the level of overspeed has to be significantly more than the normal running speed, somewhere between 75 and 100% I'd imagine.

    As has been said above the speed limits are largely for passenger comfort although having a health factor of safety isn't a bad thing either.
     
  27. mmh

    mmh Established Member

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    Indeed. Some people seem to be forgetting that the wheels on a tilting train remain perpendicular to the rail. As someone said (I think perhaps you, apologies if not) it's not like a motorbike banking.
     
  28. whhistle

    whhistle On Moderation

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    But how much was the train speeding by?
    It wasn't just 5 or 10 mph, or even the relatively nominal 25 mph we're talking about here.
    It was TWICE the speed limit (which was 80 kph).

    If a train in the UK was travelling at 250mph round Weedon I'd expect it to derail just the same.
     
  29. Bletchleyite

    Bletchleyite Veteran Member

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    Yes, there will be such a point, but whether the train is tilted or vertical will have little or no impact on that point.
     
  30. ainsworth74

    ainsworth74 Moderator Staff Member Moderator

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    Quick note. Anyone who wants to talk about the West Coast Partnership franchise please use the existing thread which can be found here. Anyone who wants to talk about new rolling stock for that franchise should use the dedicated thread which can be found here.

    This thread we are now in is for discussion the slightly broader topic of the impact of tilt on keeping trains on the track.
     
  31. bastien

    bastien Member

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    Just my attempt at an analogy, imagine a single bogie supporting a cylindrical weight. I could turn the weight by 8, 90, 180 or 360 degrees and it'd make no difference to what the bogie 'feels'.
     

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