Speed limit sign locations

Discussion in 'Infrastructure & Stations' started by Bayum, 24 Aug 2015.

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  1. Bayum

    Bayum Established Member

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    I recall (possibly incorrectly) reading that on some lines in the UK, mainline or underground I can't remember which, the driver is only allowed to accelerate once the end of the train has passed the speed limit sign.

    Why? Would it not make sense to place the speed limit sign where speed can actually increase as opposed to waiting until the back of the train has passed over it?

    Is that too simple an idea, and perhaps some sort of safety mechanism prevents this happening?
     
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  3. carriageline

    carriageline Established Member

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    I suppose it's because of the varying length of trains. Okay, you could go for the longest stock that uses the route, but what if you get a freight or engineering train?
     
  4. notadriver

    notadriver Established Member

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    Some locations are now having 4 car clear, 8 car clear etc signs to assist drivers.

    Except on cab signalled or ATP equipped lines there is no safety mechanism which prevents train drivers from exceeding speed limits. Despite this speeding is extremely rare in comparison with road vehicles. Speed limiters on heavy road vehicles do not stop them exceeding lower limits which they frequently do.
     
  5. Railsigns

    Railsigns Established Member

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    That's the main reason but it could also get very messy where there are facing points beyond the end of the speed restriction; there would have to be individual signs on each route.
     
  6. 61653 HTAFC

    61653 HTAFC Established Member

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    Presumably once in-cab signalling becomes commonplace this will no longer be a problem? I assume the ETCS level 2 will have the capability to know the length of the train being operated, and the "target speed" will only update once the entire consist is safe to accelerate?

    NB: I may have used incorrect terminology above, and indeed may have misunderstood how ETCS will operate, working on the assumption that it will be broadly similar to the system in use on HS1.
     
  7. tsr

    tsr Established Member

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    TPWS is crucial in preventing trains running over a selected limit (down to 0.1mph increments) at many higher-risk locations.
     
  8. class 9

    class 9 Member

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    A Drivers route knowledge will tell him/her where the speeds change and depending on length of train will guide them as where to accelerate.
    Yes the whole train has to be clear of any PSR/ESR or change in line speed.
     
    Last edited: 24 Aug 2015
  9. edwin_m

    edwin_m Veteran Member

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    Some freight locos have distance counters where the driver can dial in the train length then press a button as they pass the end of the restriction and it will remind them after the train has travelled its own length and they can accelerate.

    Incidentally speed restrictions at open crossings only apply until the front of the train has passed the crossing.
     
  10. notadriver

    notadriver Established Member

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    Only at the location where it's fitted - not after that. Irrelevant anyway. Train drivers don't knowingly exceed speed limits.
     
  11. Rich McLean

    Rich McLean Established Member

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    Route knowledge helps. Different drivers have different methods.

    I know how many seconds for each length of train I drive and at different speeds it takes for the train to clear the Speed Board before I apply power. I count out loud pretty much and it works.

    If you are driving fixed formations of the same traction (4 8 or 12 car), track side features can also be used.
     
  12. class 9

    class 9 Member

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    Pedantic moment now! Roads have speed limit signs, on railway it's known as line speed.
     
  13. Railsigns

    Railsigns Established Member

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    "Line speed" generally refers to the upper limit on the route, which is subject to lower "permanent speed restrictions" at specific locations. This distinction has become blurred in recent years however (modern sectional appendices no longer make a distinction between the two), and the term "permissible speed" now refers to the permitted speed at any given point on the line.

    Provision of Permissible Speed Signs
     
  14. Joseph_Locke

    Joseph_Locke Established Member

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    On Network Rail lines "Linespeed" is deprecated as all speeds should now be signed on the ground - the old SA note "maximum permissible speed unless signed otherwise" no longer applies.

    - Permissible Speed is the speed that applies to all trains
    - A Standard Differential Permissible Speed has one value for (effectively) passenger stock and one for freight.
    - A Non-standard Differential Permissible Speed applies to a limited group of rolling stock classes (e.g. MU, SP, EPS).

    You can have up to three different speed values at any given location (one of which must be the Permissible Speed), so Linespeed is a nonsense anyway.
     
  15. Llama

    Llama Established Member

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    Any driver worth their salt won't need signage to denote permissible speeds, the sectional appendix gives all the information which can be worked out accurately using mileposts and other rout features, and there are many places where PSR boards are either deliberately not placed (ie in tunnels) or deliberately placed either prior to or (more rarely) actually after the commencement of a PSR.

    There is one exception where a train may accelerate to line speed before the whole train has passed clear of a speed restriction, irrespective of train length...
     
  16. Peter Mugridge

    Peter Mugridge Established Member

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    Would that be this one in post #8?

     
  17. Llama

    Llama Established Member

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    Yes, the speed indicators approaching AOCLs, ABCLs and derivatives (AOCL+B), and also the combined speed & whistle boards approaching 'open crossings' (which are a distinct different type or crossing to AOCLs etc).
     
  18. notadriver

    notadriver Established Member

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    We don't have any of those around here. It's just a permanent speed limit board if applicable.
     
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