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What will be of interest to HN readers is the reason for, well one of, the delay. The Elizabeth line (it was called Cross Rail during development) is the first

What will be of interest to HN readers is the reason for, well one of, the delay. The Elizabeth line (it was called Cross Rail during development) is the first train line that combines both the national rail infrastructure and the London Underground. Combining the signalling systems has be an incredible complex problem. There is quite a bit online to read about it: https://www.wired.co.uk/article/crossrail-delay-logistics https://www.wired.co.uk/article/crossrail-delay-logistics https://www.crossrail.co.uk/project/our-plan-to-complete-the-elizabeth-line/signalling-and-testing# https://www.crossrail.co.uk/project/our-plan-to-complete-the...

Arbortheus 4y ago
This video also explains the problems: https://youtu.be/rtE0nlOrwGQ https://youtu.be/rtE0nlOrwGQ. In fact, it's not just the problem of combining the national rail and underground signalling systems, but there's even a third signalling system used by the Heathrow Express. So the trains will need to seamlessly transition between TPWS on the national railway lines, then CBTC in the London underground, and ETCS when going to Heathrow airport. I think this is what they call integration hell, and is responsible for delays of several years. When you consider the complexity, but also the safety-critical nature of these train signalling systems, it's no wonder it has overrun.
- steve_gh 4y ago
  Here's the basic problem. Network Rail (the UK's national rail track authority) runs a timetabled service. So they have planned where every train should be at any moment. The underground is a metro service. They are not trying to maintain an absolute timetable, but a constant interval between trains - for example if they have a problem with a train they will delay trains ahead to try and smooth the intervals between trains and stop platforms becoming dangerously crowded. So the "integration hell" is trying to integrate between services with completely different service strategies.
  - WastingMyTime89 4y ago
    These are not completely different service strategies. Both have planned where every trains should be at any moment. The difference is the amount of leeway you have to fix operational issues. You can think of the metro as a “timetabled” system where the density is so high you have no margin. When something happen you have to shift and cancel. Train lines start having exactly the same issues when they operate near capacity. Both systems have the same constraints. Trains can’t overtake. You can only have one per block. The integration difficulty is that they don’t manage blocks in the same way and use different signalling. You see the same issue on lines which are shared between SNCF and RATP in Paris. The A line used to require a driver change at the junction.
    - tialaramex 4y ago
      Right. Metro service is a passenger feature but it isn't how the service is actually delivered. As a passenger your experience during core hours of operation, and especially in the inner city, is that you descend into the system and you just take the next train, because there will be a next train. Don't run for a departing train, just wait for the next one. There actually is a timetable for all of London Underground, it's needed to roster drivers. But the only services ordinary passengers actually care about are the first ones and the last ones, Amersham to Baker Street last train is 23:22, if you're out of the city too late for that, you're not getting a tube home. When/ where there is Night Tube you probably don't care, as most people taking Night Tube aren't exactly on a tight schedule. However, the Underground is not run at capacity as you imply. There is deliberately wiggle room for "recovery" from events even at 30tph. This is one reason why the Circle Line isn't a circle any more, it makes it possible to recover more easily on that line.
      - WastingMyTime89 4y ago
        > However, the Underground is not run at capacity as you imply. There is deliberately wiggle room for "recovery" from events even at 30tph. Part of the underground is definitely run so close to that you can think of it as run at capacity. There is some wiggle room especially where it’s manually driven and during the slowest hours but nothing comparable to a typical train service. I am admittedly a lot more familiar with the French situation - I worked for the French national infrastructure manager on modernising traffic management and for TGV on their planning system - but some lines in Paris the block sec is so low even the slightest delay in doors closing will lead to cancellation and trains can’t be controlled manually.
- 7952 4y ago
  Crossrail is not part of the the London Underground and uses its own tunnels. The trains will be longer, fatter and use overhead electrification instead of the third rail used on the underground. Although the same signalling system is being installed on some of the London Underground. Outside of the central section it runs on existing mainline tracks on the Great Western and Great Eastern which both had different legacy systems.
- mcintyre1994 4y ago
  I guess they must have solved the underground/Heathrow in the past for the Piccadilly line which has multiple Heathrow stops, so I wonder how much of however that works they could re-use.
  - Doctor_Fegg 4y ago
    No - the Piccadilly Line is just plain vanilla Underground, whereas the Heathrow Express line (which the Elizabeth Line is using) is a branch off the Great Western Main Line railway.
    - mcintyre1994 4y ago
      Ah of course, Heathrow Express is another line - makes sense!
tomalpha 4y ago
It’s possible I understand this differently to the way you meant it, but other sections of the Underground have National Rail trains running on them, and visa versa. For example, the Wimbledon Branch of the District Line : https://districtdavesforum.co.uk/thread/30753/wimbledon-branch-operational-rules https://districtdavesforum.co.uk/thread/30753/wimbledon-bran... (I know it’s a forum post but that old website is a wealth of information about the District Line).
deleted 4y ago
[deleted]
iggldiggl 4y ago
> The Elizabeth line (it was called Cross Rail during development) is the first train line that combines both the national rail infrastructure and the London Underground. Combining the signalling systems has be an incredible complex problem. No, it isn't. Historically the boundary between the Underground and the rest of the "mainline" railway network used to be even fuzzier with lots of through-running, but even today a few stretches of lines served by both Underground and mainline trains remain (the Metropolitan Line north of Harrow-on-the-Hill, the outer ends of the District Line to both Richmond and Wimbledon, the Bakerloo Line north of Queen's Park). What's true though is that due to its capacity requirements, Crossrail is using a different signalling system than the rest of the railway network, which means lovely interface issues at the transition points…

Arbortheus 4y ago

This video also explains the problems: https://youtu.be/rtE0nlOrwGQ https://youtu.be/rtE0nlOrwGQ. In fact, it's not just the problem of combining the national rail and underground signalling systems, but there's even a third signalling system used by the Heathrow Express. So the trains will need to seamlessly transition between TPWS on the national railway lines, then CBTC in the London underground, and ETCS when going to Heathrow airport. I think this is what they call integration hell, and is responsible for delays of several years. When you consider the complexity, but also the safety-critical nature of these train signalling systems, it's no wonder it has overrun.

steve_gh 4y ago
Here's the basic problem. Network Rail (the UK's national rail track authority) runs a timetabled service. So they have planned where every train should be at any moment. The underground is a metro service. They are not trying to maintain an absolute timetable, but a constant interval between trains - for example if they have a problem with a train they will delay trains ahead to try and smooth the intervals between trains and stop platforms becoming dangerously crowded. So the "integration hell" is trying to integrate between services with completely different service strategies.
- WastingMyTime89 4y ago
  These are not completely different service strategies. Both have planned where every trains should be at any moment. The difference is the amount of leeway you have to fix operational issues. You can think of the metro as a “timetabled” system where the density is so high you have no margin. When something happen you have to shift and cancel. Train lines start having exactly the same issues when they operate near capacity. Both systems have the same constraints. Trains can’t overtake. You can only have one per block. The integration difficulty is that they don’t manage blocks in the same way and use different signalling. You see the same issue on lines which are shared between SNCF and RATP in Paris. The A line used to require a driver change at the junction.
  - tialaramex 4y ago
    Right. Metro service is a passenger feature but it isn't how the service is actually delivered. As a passenger your experience during core hours of operation, and especially in the inner city, is that you descend into the system and you just take the next train, because there will be a next train. Don't run for a departing train, just wait for the next one. There actually is a timetable for all of London Underground, it's needed to roster drivers. But the only services ordinary passengers actually care about are the first ones and the last ones, Amersham to Baker Street last train is 23:22, if you're out of the city too late for that, you're not getting a tube home. When/ where there is Night Tube you probably don't care, as most people taking Night Tube aren't exactly on a tight schedule. However, the Underground is not run at capacity as you imply. There is deliberately wiggle room for "recovery" from events even at 30tph. This is one reason why the Circle Line isn't a circle any more, it makes it possible to recover more easily on that line.
    - WastingMyTime89 4y ago
      > However, the Underground is not run at capacity as you imply. There is deliberately wiggle room for "recovery" from events even at 30tph. Part of the underground is definitely run so close to that you can think of it as run at capacity. There is some wiggle room especially where it’s manually driven and during the slowest hours but nothing comparable to a typical train service. I am admittedly a lot more familiar with the French situation - I worked for the French national infrastructure manager on modernising traffic management and for TGV on their planning system - but some lines in Paris the block sec is so low even the slightest delay in doors closing will lead to cancellation and trains can’t be controlled manually.
7952 4y ago
Crossrail is not part of the the London Underground and uses its own tunnels. The trains will be longer, fatter and use overhead electrification instead of the third rail used on the underground. Although the same signalling system is being installed on some of the London Underground. Outside of the central section it runs on existing mainline tracks on the Great Western and Great Eastern which both had different legacy systems.
mcintyre1994 4y ago
I guess they must have solved the underground/Heathrow in the past for the Piccadilly line which has multiple Heathrow stops, so I wonder how much of however that works they could re-use.
- Doctor_Fegg 4y ago
  No - the Piccadilly Line is just plain vanilla Underground, whereas the Heathrow Express line (which the Elizabeth Line is using) is a branch off the Great Western Main Line railway.
  - mcintyre1994 4y ago
    Ah of course, Heathrow Express is another line - makes sense!

tomalpha 4y ago

It’s possible I understand this differently to the way you meant it, but other sections of the Underground have National Rail trains running on them, and visa versa. For example, the Wimbledon Branch of the District Line : https://districtdavesforum.co.uk/thread/30753/wimbledon-branch-operational-rules https://districtdavesforum.co.uk/thread/30753/wimbledon-bran... (I know it’s a forum post but that old website is a wealth of information about the District Line).

deleted 4y ago

[deleted]

iggldiggl 4y ago

> The Elizabeth line (it was called Cross Rail during development) is the first train line that combines both the national rail infrastructure and the London Underground. Combining the signalling systems has be an incredible complex problem. No, it isn't. Historically the boundary between the Underground and the rest of the "mainline" railway network used to be even fuzzier with lots of through-running, but even today a few stretches of lines served by both Underground and mainline trains remain (the Metropolitan Line north of Harrow-on-the-Hill, the outer ends of the District Line to both Richmond and Wimbledon, the Bakerloo Line north of Queen's Park). What's true though is that due to its capacity requirements, Crossrail is using a different signalling system than the rest of the railway network, which means lovely interface issues at the transition points…