Deceleration of Jubilee Line trains at underground vs overground stations

[SWRtrainfan]

Not sure if it happens on other LU lines too, but I've noticed on the Jubilee Line at least, the trains brake more quickly on the underground sections and more slowly on the overground sections. I'm curious as to why?

 

[TRAX]

Probably because there’s more chance of a slippery rail on the overground sections ?

 

[bluegoblin7]

Exactly that. There are different braking rates for surface and tunnel sections, and this applies across the TBTC/CBTC lines.

 

[SWRtrainfan]

Ah I see, thanks!

 

[Basil Jet]

Is it also the case that the underground stations are humped, i.e. the stations are higher than the tracks either side to aid deceleration and acceleration, whereas the tracks through surface stations are generally flat?

 

[rebmcr]

Is it also the case that the underground stations are humped, i.e. the stations are higher than the tracks either side to aid deceleration and acceleration, whereas the tracks through surface stations are generally flat?

It's true on some lines, the newer ones had less freedom in their route so it fell out of fashion. Even on the lines that have it, modern timetabling demands faster progress and trains use power to maintain speed on the uphill sections designed for deceleration.

 

[xtmw]

Not sure if it happens on other LU lines too, but I've noticed on the Jubilee Line at least, the trains brake more quickly on the underground sections and more slowly on the overground sections. I'm curious as to why?

Yes this happens on the Central too

In addition on the Central there are specific brake rates in Autumn to combat adhesion problems

 

[TRAX]

Is it also the case that the underground stations are humped, i.e. the stations are higher than the tracks either side to aid deceleration and acceleration, whereas the tracks through surface stations are generally flat?

I thought this only applied to the Victoria ?

 

[stuu]

I thought this only applied to the Victoria ?

No, I think it was first done on the Central line but it might have been earlier. You can actually see it if you stand at Bond St or TCR and look along the tracks - you can see the next station and the rise and fall of the tracks (whilst remaining behind the yellow line of course).

I think it's pretty much standard in most underground lines to do that

 

[notverydeep]

Exactly that. There are different braking rates for surface and tunnel sections, and this applies across the TBTC/CBTC lines.

To add slightly more detail, surface ATO brake rates on CBTC / TBTC lines and the Central line can be set to different values, for example with lower brake rates often set during periods of poor adhesion.

 

[Sad Sprinter]

Yes I noticed the inverters seem more vocal once you get to Canning Town

 

[bramling]

Probably because there’s more chance of a slippery rail on the overground sections ?

Sort of. But the real reason is that the TBTC system has a design issue that results in the train losing communication with the system should excessive wheelslide occur, which then results in a significant delay whilst a process is followed to “re-enter” it. So a low brake rate has been chosen for the open sections to reduce the risk of this happening. It’s a problem that’s never been able to be addressed, unfortunately.

 

[boiledbeans2]

Sort of. But the real reason is that the TBTC system has a design issue that results in the train losing communication with the system should excessive wheelslide occur, which then results in a significant delay whilst a process is followed to “re-enter” it. So a low brake rate has been chosen for the open sections to reduce the risk of this happening. It’s a problem that’s never been able to be addressed, unfortunately.

Is this an issue specific to LU TBTC? Or also TBTC systems used worldwide?

== Doublepost prevention - post automatically merged: 21 Nov 2024 ==

No, I think it was first done on the Central line but it might have been earlier. You can actually see it if you stand at Bond St or TCR and look along the tracks - you can see the next station and the rise and fall of the tracks (whilst remaining behind the yellow line of course).

I think it's pretty much standard in most underground lines to do that

In fact, you can see it at one end of the Central line stations (Holland Park - Liv St). The stations were extended some time after they were built. So one end of the station is sloped as that section was originally the beginning or end of the tunnel.

 

[GFE]

Is this an issue specific to LU TBTC? Or also TBTC systems used worldwide?

== Doublepost prevention - post automatically merged: 21 Nov 2024 ==

This can be an issue on any ATO system that uses tachos on driven/braked axles to derive its exact position by the distance travelled - for example it occasionally occurs on the Central line despite various system design improvements and Leaffall season measures.
Generally a trainborne ATO system is designed to declare itself "lost" if it sees things/discrepancies that indicate its calculated position along the track to be in doubt eg Excessive wheel spin/slide (ie impractical tacho speed changes) or alternatively seeing a signal from a trackside signalling feature (eg balise, coded track boundary, position beacon) at a position too far at odds with the position it expects it in the geographic data it holds. Typically the ATO will stop the train and require its position to be reestablished (by berthing at a station or seeing unique beacons).
GPS or Doppler based ATO systems may not suffer in the same way

 

[announcements]

I thought this only applied to the Victoria ?

Only some parts. I noticed quite a decent on the southbound coming into King's Cross St. P, for instance.

 

[xtmw]

For context, the Central Line ATO system works on wheel revolutions. It will count how many revolutions there are to get to the next station. Of course in the wet the wheels spin faster so the ATO system will just stop because it gets lost. Usually the messages "LOW ADHESION" or "ATO OVERSPEED" begin piling up on the DTS. If this doesn't occur, usually the train will pull up to a station a couple cars short, so the driver will have to put the train into CM. If the starting signal is at danger they'll have to be quick!

Usually why you see lots of drivers in CM in the train, or of course the line controllers may send out an instruction to suspend ATO in a certain area.

I'm not aware how TBTC/CBTC calculates how to get to the next station, does it do this via the tags/lineside equipment?

 

[Dstock7080]

I'm not aware how TBTC/CBTC calculates how to get to the next station, does it do this via the tags/lineside equipment?

CBTC uses track mounted tags for primary and axle counters for secondary.
CBTC overspeed would result in emergency brake application to stop.

 

[boiledbeans2]

For context, the Central Line ATO system works on wheel revolutions. It will count how many revolutions there are to get to the next station. Of course in the wet the wheels spin faster so the ATO system will just stop because it gets lost. Usually the messages "LOW ADHESION" or "ATO OVERSPEED" begin piling up on the DTS. If this doesn't occur, usually the train will pull up to a station a couple cars short, so the driver will have to put the train into CM. If the starting signal is at danger they'll have to be quick!

Usually why you see lots of drivers in CM in the train, or of course the line controllers may send out an instruction to suspend ATO in a certain area.

I'm not aware how TBTC/CBTC calculates how to get to the next station, does it do this via the tags/lineside equipment?

I'm quite surprised to see that Central Line uses wheel revolutions for stopping the train. The stopping position seems quite precise. As the wheel size wears down, wouldn't it affect the distance calculations?

 

[GFE]

The Central line uses Tachos for Safety critical ATP speed monitoring and basic speed related pulse trains are also passed from the ATP to the ATO for it to determine direction, speed and distance travelled (accumulated from previous station). Yes you are right there are maintenance procedures to set/adjust to compensate for wheel change/wear. The ATO has a system of further tweaking its "wheel diameter" value based on measurements between trackside (PAC) markers at each platform stop (ie derived distance vs geographic data).
This means that the ATO distance travelled is accurate to within +/- 15m even for the longest interstation runs. This is generally OK enough for Signal stops (although the "homes" at Liv St WB and Stratford can be a bit close) and can be accommodated for in station stops once the distance to stop is adjusted once the ATO sees the PAC marker on platform entry.

 

[Backroom_boy]

I thought this only applied to the Victoria ?

I think the Victoria was the first to use a saw tooth profile with a sharp gradient just at the station to help acceleration/deceleration. Previous lines had a more gentle slope.

AFAIK Jubilee didn't bother as the thought was regen braking would be better aicbw

 

[Cricketer8for9]

I think the Victoria was the first to use a saw tooth profile with a sharp gradient just at the station to help acceleration/deceleration. Previous lines had a more gentle slope.

AFAIK Jubilee didn't bother as the thought was regen braking would be better aicbw

I’m certain the Central London Railway opened in 1900 had this. And there are certainly Victoria line stations where it doesn’t apply. Can’t speak for City and South London (though King William St was def approached upgrade). Also not sure about the 1898 Waterloo and City Railway.

 

[boiledbeans2]

[...]
This is generally OK enough for Signal stops (although the "homes" at Liv St WB and Stratford can be a bit close) and can be accommodated for in station stops once the distance to stop is adjusted once the ATO sees the PAC marker on platform entry.

Interesting. Do you mean that stopping precisely at red signals also relies on the wheel revolution counter?

As mentioned by xtmw above, wheel slips would cause the train to stop short. But if the wheel locks during an earlier braking (an earlier braking attempt, not braking for the red signal in discussion), this may cause the opposite and the train may overshoot the signal?

 

[GFE]

We are drifting off into specifics of the central line (which may not apply to the latest moving block and/or CBTC systems)
You are right that Central ATO stopping accuracy between stations (ie at signals/Marker boards) relies on the ATO determination of distance travelled from the last station; and there are no other "update/reference points" apart from the PAC markers in platforms.
However there are plenty of system features in the original system (and been retrofitted/enhanced since) to manage/compensate for the slip/slide/spin that you mention. eg Traction control and Wheel slide protection plus the ATO will typically aim to stop the train 10m prior to Signal/Marker board anyway.
Worth stressing that the positioning of trains for ATP protection is by trackside systems (not trainborne equipment position/distance determination) Ie the ATP speed limits (movement authority) are completely determined by the trackside systems (eg other train positioning known from track circuits and emergency braking distances based on ATP code max speed). The ATP Code (speed limits) then passed to physically defined areas (typically a track circuit). So if a train goes onto a track circuit and trainborne ATP "detects" a 0 speed limit (no movement authority) from that track circuit it is on, ATP will apply the emerg brakes to enforce this.

 

[Basil Jet]

and can be accommodated for in station stops once the distance to stop is adjusted once the ATO sees the PAC marker on platform entry.

Presumably the number of wheel rotations between PAC markers (what does that stand for? Are they visible or are they radio beacons?) can be used to inform the train how big its wheels are today.

 

[xtmw]

PAC = Platform ATO communicator, and yes they are visible. A small box-like shape, usually out of view from the passenger. The PAC contains information such as the CSDE for the next station, brake rate etc. IIRC it takes 7 seconds for the information to be loaded onto the train.

 

[rebmcr]

It would seem trivial to include a single unbraked wheelset within a formation, simply to provide unbiased information for the ATP computer (but probably also a useful input for the WSP computer). I wonder why this is not standard practice?

 

[GFE]

Along the same lines , it has been noted that the wheels do help clear the water from railhead + some trains have sanding facilities, so the worst slip/slide is on the the leading wheels and the situation improves as you go back down the train. The active trainborne CBTC/TBTC/ATP/ATO controllers & associated interfaces are normally in the leading car. Some systems (including 4LM ?) have the the facility for the trailing car equipment to be used - I think this was originally incorporated mainly as a backup for equipment failure, but I think it has been found useful for these adhesion issues

 

[Dstock7080]

The trainborne CBTC/TBTC/ATP/ATO controllers & associated interfaces are normally in the leading car. Some systems (including 4LM ?) have the the facility for the trailing car equipment to be used - I think this was mainly as a backup for equipment failure, but I think it has been found useful for these adhesion issues

LU CBTC uses the rear control for primary and only the front in case of failure to reset

 

[GFE]

I wasn't aware this is now the "norm", but makes sense.
I saw others statements earlier in this chain, I still suspect the LUL CBTC & TBTC trainborne equipment relies on Tachos for critical ATP & ATO related functions (speed + Distance travelled since last TAG /position reference for its own ATO driving + the status/position/occupancy reporting the train does to the control centre/safety critical interlocking (for it to decide/authorise safe movement of other trains)