GWML Electification

[Edwin_m]

Rather depends on the signal heads. The modern LED signals have proven sufficiently reliable such that maintenance requirements are almost nil. Therefore there is no need to provide ladders, walkways, cages etc or the strength to support several signal engineers working at the end of the gantry. The signal heads themselves may also be a lot lighter too.

There's also a multiplier effect because the ladders, walkways, cages etc increase the weight and wind loadings to be carried by the structure, which is therefore beefed up accordingly.  This increases the weight and wind loadings...

[The Stationmaster]

What appeared to be a wiring train noted this afternoon between Pangbourne and Basildon although on a section already wired - ? carrying out adjustments?

[Gilbert]

Works at Keynsham between Bristol and Bath today. The little I know from conversations on the bridge overlooking the site today:

• The track will end up 0.5m below current level but they need to go down a bit more during the excavation apparently.
• Platforms are not being lowered during this phase.

Edited April 2, 2016 by Gilbert

[Reorte]

Works at Keynsham between Bristol and Bath today. The little I know from conversations on the bridge overlooking the site today:

• The track will end up 0.5m below current level but they need to go down a bit more during the excavation apparently.
• Platforms are not being lowered during this phase.

Are the platforms at current standards already or will lowering the track conveniently put them at a good height?

[Gilbert]

Are the platforms at current standards already or will lowering the track conveniently put them at a good height?

I think they are currently "low" but I'm not certain

[Gilbert]

Work is scheduled to go on until 10th so I'l pop down every now and again to monitor progress

[The Stationmaster]

I do wonder about some of the track lowering which is going on as I'm plenty old enough to remember why it was raised in the first place from the late 1960s (but mainly the 1970s) onwards.  Part of it was due to needing to get a sufficient depth of ballast under cwr but most of it was to alleviate drainage problems due to what was underneath the bottom ballast,  Hence there was a mass blanketing programme all topped by new deeper ballast over much of what became the HST network in order to improve drainage, improve the stability of the roadbed and reduce the frequency of broken rails.

 

If the new lowering is accompanied by a much deeper dig into the sub-formation then probably all will be well but if it concentrates on just reducing the depth of the top ballast I wonder what will happen in a year or two?  Fortunately there doesn't seem to much, if any, lowering going on east of Didcot and even more crucially east of Reading where some of the biggest problems used to occur causing voiding, 'pumping' sleepers and broken rails.  But there has recently been clear instances of sub-formation breakdown in the Thames Valley which suggests there is still a need for care.

[Gilbert]

The ballast being dug out today seemed to be very wet and just the top layer - I don't know what is next but it looks as if they will need to dig into the sub layer to achieve the depth required. It is only Day 1 though!

Edited April 2, 2016 by Gilbert

[phil-b259]

I do wonder about some of the track lowering which is going on as I'm plenty old enough to remember why it was raised in the first place from the late 1960s (but mainly the 1970s) onwards. Part of it was due to needing to get a sufficient depth of ballast under cwr but most of it was to alleviate drainage problems due to what was underneath the bottom ballast, Hence there was a mass blanketing programme all topped by new deeper ballast over much of what became the HST network in order to improve drainage, improve the stability of the roadbed and reduce the frequency of broken rails.

 

If the new lowering is accompanied by a much deeper dig into the sub-formation then probably all will be well but if it concentrates on just reducing the depth of the top ballast I wonder what will happen in a year or two? Fortunately there doesn't seem to much, if any, lowering going on east of Didcot and even more crucially east of Reading where some of the biggest problems used to occur causing voiding, 'pumping' sleepers and broken rails. But there has recently been clear instances of sub-formation breakdown in the Thames Valley which suggests there is still a need for care.

Modern practice seems to be to add geotectile membrane to areas where the underlying earthworks are suspect to prevent ballast becoming contaminated. Was this technique used back in the day by BR or is the technology more recent. I can imagine that if it was simply a case of achieving a good base by stone alone the depth of the base stoning / balls sting must be grater than if membrane is used as well. Edited April 2, 2016 by phil-b259

[The Stationmaster]

Modern practice seems to be to add geotectile membrane to areas where the underlying earthworks are suspect to prevent ballast becoming contaminated. Was this technique used back in the day by BR or is the technology more recent. I can imagine that if it was simply a case of achieving a good base by stone alone the depth of the base stoning / balls sting must be grater than if membrane is used as well.

 

That is exactly what is involved in blanketing Phil - a sort of geotextile (which I think might have varied a bit over the years) and sand.  The idea is twofold - firstly to help drainage and secondly to stop the sub-formation pumping upwards as the track moves up & down under traffic and potentially creates voids/crushes the top ballast.

 

This, the pale patch in the Up Relief - the further line - is what happens when the sub-formation starts to pump upwards.  Notice that it is right next to a fishplated block joint (which might have been removed by now - the picture was taken at the end of November and there is an axle counter head in there under all that pale coloured muck) so the presence of the joint might have contributed but it would be a good candidate as a spot where a rail might break as a consequence of the pumping action and the amount of float in the track.  Back in the early days of cwr on the GWML and before blanketing and deep ballasting you could see several spots like this in less than a mile in Sonning Cutting - on both of the Main Lines.

 

[St. Simon]

DSCF8845rd.jpg

Ah, wet beds, so fun to dig out by hand!

 

Simon

[Gilbert]

There seemed to be quite a bit of something that looked like a membrane coming out with the ballast that was being removed.

[Northroader]

You get these with local spots where the ground is soft, and the track foundation is thin enough to allow movement with the weight of the train passing over. It ends up with a pool of water and mud, which helps to show up the problem. By then the track can be moving up or down a couple of inches or more. As these days everything is running on bogies or with "continental" suspension, it isn't quite the problem it used to be with traditional goods wagons and their suspension. The rails needn't deflect both together the same amount, so one stays higher than the other, if you get on your hands and knees and look across the track, any discrepancy between the height of the rails is termed the twist. Derby research gave a figure of so many degrees, which I forget, beyond which it was potentially dangerous. So on the one hand there's the danger of broken rails, otherwise the risk of "plain line derailment" to which faulty wagon suspension and excessive speed would also contribute. Happy days! I haven't been that way recently, but I found the tracks across Wandsworth Common were a good place to see these spots.

Edit added: I was racking my brain, what were they called? Just come to me- "voids" -memories do get bad as you grow older!

Edited April 3, 2016 by Northroader

[Edwin_m]

Works at Keynsham between Bristol and Bath today. The little I know from conversations on the bridge overlooking the site today:

• The track will end up 0.5m below current level but they need to go down a bit more during the excavation apparently.
• Platforms are not being lowered during this phase.

I suspect you were mis-informed on the second point.  A new platform will be nominally 3ft above rail but anything higher would foul passing freight trains.  If the current elevation is acceptable with 0.5m of lowering then the platforms must have been not much more than a foot above rail level before!  I think also the removal of the coping stones suggests that the platforms will be altered - but perhaps by "this phase" they mean that they won't try to do it until the day after they finish the track? 

[Fat Controller]

You get these with local spots where the ground is soft, and the track foundation is thin enough to allow movement with the weight of the train passing over. It ends up with a pool of water and mud, which helps to show up the problem. By then the track can be moving up or down a couple of inches or more. As these days everything is running on bogies or with "continental" suspension, it isn't quite the problem it used to be with traditional goods wagons and their suspension. The rails needn't deflect both together the same amount, so one stays higher than the other, if you get on your hands and knees and look across the track, any discrepancy between the height of the rails is termed the twist. Derby research gave a figure of so many degrees, which I forget, beyond which it was potentially dangerous. So on the one hand there's the danger of broken rails, otherwise the risk of "plain line derailment" to which faulty wagon suspension and excessive speed would also contribute. Happy days! I haven't been that way recently, but I found the tracks across Wandsworth Common were a good place to see these spots.

The effect when vehicles with certain types of suspension (the type based around coil, rather than leaf, springs) pass over these can be spectacular- rather like standing on one side of a paving slab that's not sitting flat. I've seen wagons with an almost vertical stripe on the body above each axle, wher the displaced gunk has sprayed upwards. The WCML around Rugby used to be notorious for these 'wet spots'; even though the track had been relaid during electrification, only the top layer of ballast had been relaid, so the effects of 100 years or so of trains passing was still evident in the layers below. IIRC, the places where 'pumping' occured were roughly where old rail-joints had been, so they'd be 45' or 60' apart, depending what length rail had been there previously. When the WR upgraded the Badminton line in 1975, they removed all the ballast, and the material immediately below it, then laid sand, followed by geotextile, and then ballast to the reqisite level. This seems to have become standard practice elsewhere, at least on those relaying jobs I've observed, such as the upgrade of the lines towards Folkestone.

The dips at the rail-joints produced an effect called 'cyclic top',I believe, which would cause the suspension of wagons (noticeably those with short wheel-bases and plain suspension) to 'hunt'; in the late 1960s and early 1970s, there were a spate of 'plain line' derailments on long-welded track attributable to this, and maximum speeds for short-wheelbase wagons were reduced dramatically.

Edited April 3, 2016 by Fat Controller

[Captain Kernow]

I suspect you were mis-informed on the second point.  A new platform will be nominally 3ft above rail but anything higher would foul passing freight trains.  If the current elevation is acceptable with 0.5m of lowering then the platforms must have been not much more than a foot above rail level before!  I think also the removal of the coping stones suggests that the platforms will be altered - but perhaps by "this phase" they mean that they won't try to do it until the day after they finish the track? 

The platform works at Keynsham will commence a bit later in this blockade and are being done by Hochtief. It's not practical to have two different contractors working so close to each other (track laying and platform civil engineering).

 

There is another major track relaying site in this blockade, at Bath Goods, where the S&C renewals team are renewing all the points (two crossovers and the entrance and exit points from the Down Goods Loop, plus any associated traps).

 

I am have arranged to visit the Keynsham site tomorrow, (in my capacity as an independent co-author of another future publication), and then the S&C site later in the week.

[Gilbert]

Here are some photos from today - first track relaid. It's very deceptive - it doesn't look 0.5m lower.

[Geoff Endacott]

Here are some photos from today - first track relaid. It's very deceptive - it doesn't look 0.5m lower.

 

Maybe it has only been moved part of the way and the rest will be done by moving the sky up.

 

Geoff Endacott

[GWRPhil]

Maybe it has only been moved part of the way and the rest will be done by moving the sky up.

 

Geoff Endacott

What difference would lifting your satellite box higher up make?

[Geoff Endacott]

What difference would lifting your satellite box higher up make?

 

It would make it further away from the ground, of course.

 

Geoff Endacott

[GWRPhil]

Ah. Didn't look at it from that angle. Perhaps that works with the track laying too. They're relying on the fact that from platform level it looks like it's been lowered the appropriate amount.

[david.hill64]

The dips at the rail-joints produced an effect called 'cyclic top',I believe, which would cause the suspension of wagons (noticeably those with short wheel-bases and plain suspension) to 'hunt'; in the late 1960s and early 1970s, there were a spate of 'plain line' derailments on long-welded track attributable to this, and maximum speeds for short-wheelbase wagons were reduced dramatically.

Cyclic top causes (and is caused by) vehicles to bounce vertically rather than hunt - hunting is rapid lateral oscillation of wheelsets usually caused when the conicity (the effective angle between the wheel and the rail) becomes too high. Even after the 70's there were many problems with UIC wagons suffering plain line derailments on track within maintenance tolerances. BR had worked out that it was cheaper to design vehicle suspensions properly and let track deteriorate more before maintenance. Unfortunately the continental approach has always been to try to maintain rails to billiard table top flatness, in which case vehicle suspensions can be primitive. This also led to problems with passenger vehicle design, notably the IC225 where SIG never really managed to get the same ride quality for a mark 4 coach as BR achieved with the BT10 bogie on a mark 3. Siemens and Hitachi also had problems with Desiros and 395s.

 

Cyclic top can be initiated at a wet spot especially when trains pass over at similar speeds. The suspension's natural reaction from coping with sudden dip puts high dynamic force into the rail at some point beyond the dip. This gets reinforced by following vehicles until another dip appears. the effect is self propagating as the new dip that is formed then acts as a formation point for a following dip. eventually you get to the point where resonance occurs in the suspension and wheels become totally unloaded, at which point derailment becomes a possibility.

[Titan]

As shown here, for once the title is not an exaggeration - stills from the footage was used in an RAIB report in to an actual derailment with these wagons due to cyclic top. It could be seen that at the most severe point of bounce one wagon was approximately one foot above the adjacent one. I suspect that some of the bangs you can hear as the train passes are wheels leaving the rail and luckily falling back down on them...

 

https://www.youtube.com/watch?v=L4ntnt4DL60

[flapland]

Wow, that looked scary and well done to the guy for reporting it. 

[ess1uk]

As shown here, for once the title is not an exaggeration - stills from the footage was used in an RAIB report in to an actual derailment with these wagons due to cyclic top. It could be seen that at the most severe point of bounce one wagon was approximately one foot above the adjacent one. I suspect that some of the bangs you can hear as the train passes are wheels leaving the rail and luckily falling back down on them...

 

https://www.youtube.com/watch?v=L4ntnt4DL60

Where that filmed?