How does Network Rail work out max speeds?

Discussion in 'Infrastructure & Stations' started by markymark2000, 26 Nov 2019.

  1. markymark2000

    markymark2000 Member

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    I'm sure I speak on behalf of most people when I say that going through some areas, it feels like the speed limit is quite low and could be increased. Sometimes just 5mph extra can decrease journey times by upto 5 minutes which is a big improvement.

    How does Network Rail establish what a max speed for sections of track are and how often are these evaluated?

    I use the example of the SWML from Woking to Clapham Jct. The majority of the line is 90 slowing to 75 at Raynes Park. Why couldn't this be 100mph? It's relatively straight, the fast lines have no platforms to contend with, no level crossings.

    Another thing is that some of the track access consultations for services on the WCML have asked for increases to speeds for non tilting trains. 30mph speed differences in some cases make a big difference
     
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  3. swt_passenger

    swt_passenger Veteran Member

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    Quick answer will be that the speeds are brought down gradually for the Clapham Jn bottleneck. The thorough answer will be far more complex...
     
  4. Darandio

    Darandio Established Member

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    Braking distance in consideration to the signal spacing would also be a factor I imagine, some of the sections are quite close together.
     
  5. swt_passenger

    swt_passenger Veteran Member

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    It’s all a bit academic on this stretch because AIUI any speed increase would simply raise headways and reduce peak frequency?
     
  6. Darandio

    Darandio Established Member

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    Probably yes, fair point.
     
  7. markymark2000

    markymark2000 Member

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    Is that not then the point of having flashing signals, double yellow, single yellow.

    Speed increase would surely increase capacity as trains can clear sections quicker and increase the potential TPH on the section.
     
  8. swt_passenger

    swt_passenger Veteran Member

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    Not correct, I’m afraid.
     
  9. headshot119

    headshot119 Established Member

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    Flashing signals are purely used for diverging routes, so ignore them for increasing headways (at a high level anyway). If the signals have been sighted for say 75mph running, there may be issues with drivers having enough time to sight the aspect when adding an extra 25mph to the line speed. There may also not be sufficient distance between the signals to brake from 100mph down to 0mph on encountering the double yellow.

    But you need to factor in the increased time to get to line speed, and the increased time to brake to a stand etc.
     
  10. hooverboy

    hooverboy Member

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    I think there's a lot of factors in play.
    condition/type of track and thermals.
    condition/type of rolling stock
    minimum curve radius/cant
    signal sighting/distance/visibility
    environmental factors. ie fenland being covered in mist at night and turning into a skating rink in the morning...same example for frozen points etc.
    crossings and other hazards.

    the safe option is what's chosen in most cases I think(ie high RA freight diagram taking a mile to stop. even on a passenger route.)
     
  11. The Planner

    The Planner Established Member

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  12. Snow1964

    Snow1964 Member

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    It used to be 65mph in from New Malden until the resignalling (in about 1971). The 75mph is due to the signal spacing (there are a lot of trains once branches have joined at Hampton Court Junction, New Malden and Raynes Park

    The signals are spaced approx 1050-1100 feet (320-335m) apart. This is about the minimum for a 12 car (20m) train with a 200 feet overlap behind. When there are problems not uncommon to see trains waiting at every signal, or both fast and slow lines

    To increase speed either need to space signals further or add 5th aspect
     
  13. edwin_m

    edwin_m Veteran Member

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    Yes, but the spacing between trains depends on the braking distance, which in turn depends on the square of the speed. So above a certain speed (typically around 50mph) this effect outweighs others and reduces capacity,
     
  14. Ianno87

    Ianno87 Established Member

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    On 4 aspect signalling, optimum throughput occurs at about 70mph.

    3 aspect it works out at about 25-40mph or so.
     
  15. Colin1501

    Colin1501 Member

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    Woking to Clapham Junction is just over 20 miles. How much time would be saved by increasing the line speed from 90mph to 100mph over this stretch, especially when you factor in the distance required to continue acceleration from 90mph to 100mph?
     
  16. Wychwood93

    Wychwood93 Member

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    I think that, IIRC, it was 60 in from New Malden in the late 60's - from my own timing experiences at the time this was perhaps a suggestion of the speed they should be doing. Amongst other stock, the 4-REPs may have mustered a touch more than 60. My own logs from that time would see instances of 75 - no data recording. On another tack - quite why I am mentioning this, with regard to my home line, when the OP asked a sensible question...…….. forums - why stick to the point!
     
  17. big all

    big all On Moderation

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    the other factor is different speeds and different track formations require different levels off patrolling and maintenance
     
  18. The Planner

    The Planner Established Member

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    A minute once you factor in the acceleration.
     
  19. Bald Rick

    Bald Rick Established Member

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    Thanks mate. IOU one :|

    Good grief, this again. There are over 80 separate factors that influence linespeed. Not all apply to every stretch of line, but many do. If linespeed is to be raised, every factor has to checked and signed off by the relevant professional (engineer, operations risk manger, etc). Linespeeds are not routinely reviewed - if there is a reason to consider speeds on a given stretch of line then a special exercise is undertaken.

    I can’t think of anywhere on the passenger network where a 5mph improvement would give 5 minutes. Bear in mind that upgrading the WCML between Rugby and Euston from a ruling 110mph to a ruling 125mph saved 5 minutes. (The rest came from the trains’ better performance, and greatly improved layouts at Euston and Rugby, each of which saved a minute)

    The difference between 90 and 100 is 4 seconds a mile. The 90 starts just south of a Surbiton at the 12.5 milepost (85mph on the up from Esher). Woking is a shade under 12 miles from Surbiton, allow a bit for acceleration / braking, and for trains not stopping at Woking it will be around 40 seconds. Of course there already is a stretch of 100 on the up from Woking to Byfleet (which actually starts back at Basingstoke).

    Linespeed between Surbiton and Clapham is 80 or 75mph partly because of the signal spacing necessary to achieve 2 minute headways. I also have a hunch that because of the traffic intensity, it helps with power draw.
     
  20. 30907

    30907 Established Member

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    So to save 5 minutes by an increase of 5mph (the OP's figures) you have to cover getting on for 200 miles.

    Low speed pointwork is worth doing though - I was checking an old sectional appendix and a current one earlier today, and it's interesting how some junctions have had the odd 5mph extra factored in even when there has been no change to the layout.
     
  21. Bald Rick

    Bald Rick Established Member

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    For that extra 5mph at junctions, there will have been a change to the layout, just not to the untrained eye. The function of the junction may have stayed the same, but the precise location and geometry of the points will have changed. The change of rail type from 113A to NR60 is a factor here.

    However I agree that chasing 5 or 10mph at low speed junctions is worth chasing, however it is not always possible, particularly in areas of multiple complex crossovers and/or electrification.
     
  22. AM9

    AM9 Established Member

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    Hasn't this thread gone the same way as a discussion within the Blackpool electrification thread where a couple of posters were complaining that the upgraded line didn't have maximum speed of around 100 mph, (just because the electric trains could go that fast). It was pointed out that the real time improvements were to be achieved by speeding up the slow bits rather than thrashing up to the max. for a couple of miles and then slamming on the brakes. As for any busy line, it's the discontinuities in speed that eats up the paths and slows all trains down thereby reducing capacity on the line.
     
  23. choochoochoo

    choochoochoo Member

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    What is the tolerance built into speed limits ? I imagine it depends partly on the train type. If a PSR is set to 70mph, what would be the fastest a train could travel in it ? Is there a safety margin built in for accidental overspeeds ?
     
  24. Bald Rick

    Bald Rick Established Member

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    Yes, but it varies significantly depending on the limiting factor. Some are straight engineering, others are more engineering judgement, whilst some are risk factors.
     
  25. thenorthern

    thenorthern Established Member

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    The type of train used is a factor as well.

    I know the Mid Cheshire Line has speed limits in certain sections that could be lifted if Northern agreed not to use Pacer trains on that line, its a case of some paperwork would need to be filled out and some new signs would need to be replaced however Northern hasn't been able to guarantee not to use pacers on that line so the speed limits are still in place.
     
  26. The Planner

    The Planner Established Member

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    Totally agree. Some junctions where you have freight diverging, if sped up would make quite a difference. However, unless that can get you extra capacity and not just performance as a result they just don't make it to reality.
     
  27. The Planner

    The Planner Established Member

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    That sounds odd to me, what characteristics of a Pacer are causing that?
     
  28. Bald Rick

    Bald Rick Established Member

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    That would be quite unusual, albeit not unprecedented. For a speed limit to apply to all types of train, when there is only one type of train affected, would usually be down to axle load, braking performance or loading gauge. In the case of pacers, it can only be the former, and even then I’d be surprised if a Pacer axle load is greater than a loco hauling passenger coaches. This could be covered by a special instruction in the sectional appendix, but in this case doesn’t appear to be.

    Is there a specific example?
     
  29. markymark2000

    markymark2000 Member

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    The lower the speed to start with, a 5mph increase over relatively long distances can give big time savings. The example of the SWML would not deliver a 5 minute saving I admit. It was a figure of what could be achieved in one or two circumstances. Any speed increase though is a journey time reduction and in some areas of high traffic so based on no extra TPH, it is a bit more leeway for delays to be caught up (If a project delivers 2 minute journey time reduction, accommodate 1 min for the passenger timetable and 1 min for performance allowance).
     
  30. ComUtoR

    ComUtoR Established Member

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    We have had a few reductions of speed recently on our patch. I'd love to know why that happened, there seems to be no rhyme or reason why the speeds got reduced. One, quite infamously, has changed and now adds a minute to the next stop and nothing was changed to the track, just a change of speed.

    Some of our recent changes have been placement of the PSR board. They have been moved from track to signal. This extends the distance where a PSR changes, which is also reducing the speed. I'd also love to know what has changed for PSRs to be placed on Signals rather than traditional posts in the ground :/
     
  31. dosxuk

    dosxuk Member

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    Sounds like someone mixing up speed limits and timing loads.
     

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