Trowse swing bridge - maximum capacity?

Discussion in 'Infrastructure & Stations' started by Adrian1980uk, 16 Mar 2019.

  1. Adrian1980uk

    Adrian1980uk Member

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    What do we reckon is the max number of tph over that 100m section... lots of people talk about it as a pinch point and it is but that short distance is it really a big issue
     
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  3. edwin_m

    edwin_m Veteran Member

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    For a train to have a clear run through the single line, however short it is, the signal has to be cleared long enough beforehand that it doesn't have to slow down. This is probably three or four minutes before the train actually passes, depending on details of the signalling, and the signal can't be cleared for the other direction during this time. In theory trains could then be timetabled to pass (in one direction or the other) every three or four minutes, but in practice a gap will be added particularly between trains in opposite directions so if one is a bit late it doesn't delay another one. So you're probably down to about eight trains per hour or one every 7.5min, total for the two directions*. And timetabling issues elsewhere on their routes may make it impossible to schedule the trains at optimum times, in which case the actual capacity is less than this.

    A longer single line would be worse because the time taken for the train to pass over it has to be taken into account too, but even a short single line is a significant restriction on route capacity.

    *These are typical numbers for a short single line. The actual numbers for Trowse will be in the Timetable Planning Rules on the NR website.
     
    Last edited: 16 Mar 2019
  4. hwl

    hwl Established Member

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    The comparatively low speed in the area (40mph) also makes it an issue as the transit time for that "short" section is surprisingly long!
     
  5. Class 170101

    Class 170101 Established Member

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    Three minute Jn margin and three minute headway for Trowse Swingbridge.
     
  6. AndrewE

    AndrewE Established Member

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    For my it's blindingly obvious: No matter how clever or sparse the timetable is, if an inbound train is late it may conflict with a departure. So do you hold the inbound train and make it later still? (perhaps jeopardising connections and adding maybe an hour to some people's journeys...)
    Or do you let it in - but then your departure is delayed, which might have repercussions across the network?
    This is why single-lead junctions were a false economy, to say nothing about the safety risk. If only accountants hadn't been invented! They can't put a value on resilience but they think they know the cost of everything!
     
  7. John Webb

    John Webb Established Member

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    I don't know how often boats need the bridge to be swung - but that would be another potential reduction in train movements.
     
  8. The Lad

    The Lad Member

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    I'm not sure it can be swung at the moment.
     
  9. edwin_m

    edwin_m Veteran Member

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    Usually better to hold the inbound train in my opinion. It will have some time in the terminus which will probably be enough to recover delay, whereas the outbound train will definitely be late.

    I've never really understood the cost saving claimed for single lead junctions, at least where there is room for a "ladder" junction without a diamond. Both have four sets of points so it seems to me the only saving is a hundred metres or so of plain line. And junctions are precisely where most capacity is needed, which usually happens when trains to and from the same route are timetabled to pass each other on the junction.
     
  10. AndrewE

    AndrewE Established Member

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    That may be a legitimate answer for an operator, but unfortunately it can often be the wrong one for a passenger who was expecting to make a connection. Which is why any "capacity enhancement" fund (when we get one) should also undo silly rationalisations like this, besides addressing other resilience issues too.
     
  11. swills

    swills Established Member

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    Down IC takes pref over Up IC (up to 12 late from Diss)
     
  12. Adrian1980uk

    Adrian1980uk Member

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    Clearly its logical to hold the inbound and ideally a 2 track swing bridge would be much more resilient.. I fear the cost will make it just a pipe dream.
     
  13. Alfie1014

    Alfie1014 Member

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    The added Norwich in 90 services compound this a bit come May, at around 09:00 there will be 4 consecutive trains at almost minimum headways over the bridge.

    WTT pass London Liverpool Street 1P06 LE Norwich 0854
    WTT pass Norwich 1R74 EM Liverpool Lime Street 0857
    WTT pass Norwich 9P91 LE London Liverpool Street 0901
    WTT pass Norwich 1P23 LE London Liverpool Street 0904

    and later in the day if the loco off the North Walsham tanks is returning light engine to Ipswich there are 5 movements.

    WTT pass Norwich 1R60 EM Manchester Piccadilly 1657
    WTT pass Norwich 9P93 LE London Liverpool Street 1701
    WTT pass Norwich 1P55 LE London Liverpool Street 1704
    WTT pass Liverpool Lime Street 1L10 EM Norwich 1707
    WTT pass North Walsham Gbrf FRGT ZZ Ips Rc Gbrf 1710
     
  14. 306024

    306024 Established Member

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    Clearly it isn’t :s. The down train gets priority if it has a chance of making PPM.

    Three up trains dependent on the right time running of 1P06 07.00 LST-NRW. Hmmm.... Can see 1P06 being hung out to dry if it’s a couple of minutes down.
     
    Last edited: 17 Mar 2019
  15. 30907

    30907 Established Member

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    Which is why (along with the safety issue - Bellgrove) they went out of fashion. Not sure when the last was installed (apart from Newton in 1990-ish?).
     
  16. edwin_m

    edwin_m Veteran Member

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    Indeed. I'm more questioning why so many were installed in the first place.
     
  17. John Webb

    John Webb Established Member

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    Looking at photos of the bridge on the Geograph website, I gather it was installed circa 1986 as part of the electrification process; it was also built alongside the old bridge, which was then removed. I assume both cost and site restraints played their part in it not being built as a double track bridge.
     
  18. AndrewE

    AndrewE Established Member

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    Golborne junction on (& off) the WCML from the main Liverpool - Manchester line is single-lead & wasn't done very long ago. I could hardly believe it when I saw it. http://www.opentraintimes.com/maps/signalling/warrington#LINK_1
     
  19. Bald Rick

    Bald Rick Established Member

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    Planning headway is 3 mins as others have said. Therefore capacity is 20tph. However I’ve been in Trowse ‘box and seen trains across it, in opposite directions, with headways of less than 2 minutes.
     
  20. Adrian1980uk

    Adrian1980uk Member

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    So capacity of 20 tph would be plenty for the foreseeable future as were talking 6 or 7 but I guess at peak times it could be an issue
     
  21. edwin_m

    edwin_m Veteran Member

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    Pretty sure it's been like that since the re-signalling in the 70s. It was relaied a few years back but I think that was like for like.

    20 (total of both directions) probably isn't continuously achievable in practice because if a train is delayed there is no opportunity to recover. But 6 or 7 is probably OK. I read on here somewhere there was a strategy document for GEML which said there were much bigger problems than Trowse.
     
  22. The Lad

    The Lad Member

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    But if it needed to be swung for a boat that would make a big hole in that 20 tph.
     
  23. swills

    swills Established Member

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    swung for a boat ! LOL :) :)
     
  24. 306024

    306024 Established Member

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    Only 20tph if all 20 trains are going in the same direction, and they would have to alternate coming from Thetford and Diss on the down, which I’d suggest is unlikely :).

    It takes just over half a minute to occupy the single line over the swing bridge at linespeed of 30mph. Add time to reset the route you would call this one minute in total. Using the planning rules running an up train then a down train alternately therefore reduces this theoretical capacity to:

    00-01 up, 04-05 dn, 08-09 up, 12-13 dn, 16-17 up, 20-21 dn etc.

    That’s a total of 15tph, but on a 60 minute clock face pattern rather than 30 minutes. Flighting trains improves that slightly. A three minute junction margin at the London end seems reasonable, but could possibly be reduced at the Norwich end of the bridge depending on how tight you wish to plan the timetable. However for some reason there is not much appetite for tightening timetable planning rules at the moment. You can get a train off the Thetford line closer to the bridge if you run an up London train over the down line to Lakenham, but that takes about half a minute longer to get to Diss then. Norwich in 90.5 hasn’t got the same ring to it.
     
  25. swills

    swills Established Member

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    There is quite a good documentary either in the East Anglian Film Archive, or the BFI made by Anglia TV about the Electrification of the GEML Norwich to Colchester, shows the bridge being demolished, some scary work by the OHL dept putting up wires, and a quick view of Colchester PSB, seemingly a LOT quieter in those days :)

    EAFA also have a film about 'THE ANGLIAN' in the 80's from Yarmouth to Liverpool Street.
     
  26. swills

    swills Established Member

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    Would not like to see it any less than 3 mins, it's tight enough now with the 1Rxx at 57min dep, and the 1Pxx dep at 00. when Ni90 starts we are sure that the train following the 9Pxx will get a signal check at CO546 at Lakenham, although 1P23 is allowed more time to Trowse from the Station.
     
  27. 306024

    306024 Established Member

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    Quite agree, was only going on Bald Rick’s observations. A check at CO546 would kill any chance of making Diss on time. Perhaps 1P23 should start at 09.04, providing it can still get through Witham ahead of the up Braintree. Incidentally have you noticed 1P23 loses the Manningtree call, with the Harwich branch unit running to Colchester to maintain a connection?
     
  28. Bald Rick

    Bald Rick Established Member

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    I’m not suggesting no thst capacity actually should be 20tph, but that in theory it could be.

    @swills, as mentioned above I’ve seen trains in opposite directions in under 2 minutes from
    Trowse Box - an EMT ex Thetford, with a London coming the other way. The stars were aligned obviously. (And I got to swing the bridge ;))
     
  29. Class 170101

    Class 170101 Established Member

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    Both 1P23 and 1P55 have (1) approaching Trowse Jn according to Real Time Trains. Incidently 1P55 will have to run (as scheduled) to Lakenham if its not to delay 1L10 on its approach to Trowse Jn.
     
  30. edwin_m

    edwin_m Veteran Member

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    Was the second one checked by signals on the approach?
     
  31. Adrian1980uk

    Adrian1980uk Member

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    Yes I was 6 / 7 when it was done so don't remember much... although crown point had an open day in 87 I think which my parents took me to.

    We have to remember that was during the decline of the railways so it probably made sense to build single track. In those days too the solution to more capacity was generally another coach on the train rather than extra trains
     

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